arm64: fpsimd: Generalise context saving for non-task contexts

[thirdparty/linux.git] / arch / arm64 / kernel / fpsimd.c

/*
 * FP/SIMD context switching and fault handling
 *
 * Copyright (C) 2012 ARM Ltd.
 * Author: Catalin Marinas <catalin.marinas@arm.com>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <linux/bitmap.h>
#include <linux/bottom_half.h>
#include <linux/bug.h>
#include <linux/cache.h>
#include <linux/compat.h>
#include <linux/cpu.h>
#include <linux/cpu_pm.h>
#include <linux/kernel.h>
#include <linux/linkage.h>
#include <linux/irqflags.h>
#include <linux/init.h>
#include <linux/percpu.h>
#include <linux/prctl.h>
#include <linux/preempt.h>
#include <linux/prctl.h>
#include <linux/ptrace.h>
#include <linux/sched/signal.h>
#include <linux/sched/task_stack.h>
#include <linux/signal.h>
#include <linux/slab.h>
#include <linux/sysctl.h>

#include <asm/esr.h>
#include <asm/fpsimd.h>
#include <asm/cpufeature.h>
#include <asm/cputype.h>
#include <asm/simd.h>
#include <asm/sigcontext.h>
#include <asm/sysreg.h>
#include <asm/traps.h>

#define FPEXC_IOF       (1 << 0)
#define FPEXC_DZF       (1 << 1)
#define FPEXC_OFF       (1 << 2)
#define FPEXC_UFF       (1 << 3)
#define FPEXC_IXF       (1 << 4)
#define FPEXC_IDF       (1 << 7)

/*
 * (Note: in this discussion, statements about FPSIMD apply equally to SVE.)
 *
 * In order to reduce the number of times the FPSIMD state is needlessly saved
 * and restored, we need to keep track of two things:
 * (a) for each task, we need to remember which CPU was the last one to have
 *     the task's FPSIMD state loaded into its FPSIMD registers;
 * (b) for each CPU, we need to remember which task's userland FPSIMD state has
 *     been loaded into its FPSIMD registers most recently, or whether it has
 *     been used to perform kernel mode NEON in the meantime.
 *
 * For (a), we add a fpsimd_cpu field to thread_struct, which gets updated to
 * the id of the current CPU every time the state is loaded onto a CPU. For (b),
 * we add the per-cpu variable 'fpsimd_last_state' (below), which contains the
 * address of the userland FPSIMD state of the task that was loaded onto the CPU
 * the most recently, or NULL if kernel mode NEON has been performed after that.
 *
 * With this in place, we no longer have to restore the next FPSIMD state right
 * when switching between tasks. Instead, we can defer this check to userland
 * resume, at which time we verify whether the CPU's fpsimd_last_state and the
 * task's fpsimd_cpu are still mutually in sync. If this is the case, we
 * can omit the FPSIMD restore.
 *
 * As an optimization, we use the thread_info flag TIF_FOREIGN_FPSTATE to
 * indicate whether or not the userland FPSIMD state of the current task is
 * present in the registers. The flag is set unless the FPSIMD registers of this
 * CPU currently contain the most recent userland FPSIMD state of the current
 * task.
 *
 * In order to allow softirq handlers to use FPSIMD, kernel_neon_begin() may
 * save the task's FPSIMD context back to task_struct from softirq context.
 * To prevent this from racing with the manipulation of the task's FPSIMD state
 * from task context and thereby corrupting the state, it is necessary to
 * protect any manipulation of a task's fpsimd_state or TIF_FOREIGN_FPSTATE
 * flag with local_bh_disable() unless softirqs are already masked.
 *
 * For a certain task, the sequence may look something like this:
 * - the task gets scheduled in; if both the task's fpsimd_cpu field
 *   contains the id of the current CPU, and the CPU's fpsimd_last_state per-cpu
 *   variable points to the task's fpsimd_state, the TIF_FOREIGN_FPSTATE flag is
 *   cleared, otherwise it is set;
 *
 * - the task returns to userland; if TIF_FOREIGN_FPSTATE is set, the task's
 *   userland FPSIMD state is copied from memory to the registers, the task's
 *   fpsimd_cpu field is set to the id of the current CPU, the current
 *   CPU's fpsimd_last_state pointer is set to this task's fpsimd_state and the
 *   TIF_FOREIGN_FPSTATE flag is cleared;
 *
 * - the task executes an ordinary syscall; upon return to userland, the
 *   TIF_FOREIGN_FPSTATE flag will still be cleared, so no FPSIMD state is
 *   restored;
 *
 * - the task executes a syscall which executes some NEON instructions; this is
 *   preceded by a call to kernel_neon_begin(), which copies the task's FPSIMD
 *   register contents to memory, clears the fpsimd_last_state per-cpu variable
 *   and sets the TIF_FOREIGN_FPSTATE flag;
 *
 * - the task gets preempted after kernel_neon_end() is called; as we have not
 *   returned from the 2nd syscall yet, TIF_FOREIGN_FPSTATE is still set so
 *   whatever is in the FPSIMD registers is not saved to memory, but discarded.
 */
struct fpsimd_last_state_struct {
        struct user_fpsimd_state *st;
        bool sve_in_use;
};

static DEFINE_PER_CPU(struct fpsimd_last_state_struct, fpsimd_last_state);

/* Default VL for tasks that don't set it explicitly: */
static int sve_default_vl = -1;

#ifdef CONFIG_ARM64_SVE

/* Maximum supported vector length across all CPUs (initially poisoned) */
int __ro_after_init sve_max_vl = -1;
/* Set of available vector lengths, as vq_to_bit(vq): */
static __ro_after_init DECLARE_BITMAP(sve_vq_map, SVE_VQ_MAX);
static void __percpu *efi_sve_state;

#else /* ! CONFIG_ARM64_SVE */

/* Dummy declaration for code that will be optimised out: */
extern __ro_after_init DECLARE_BITMAP(sve_vq_map, SVE_VQ_MAX);
extern void __percpu *efi_sve_state;

#endif /* ! CONFIG_ARM64_SVE */

/*
 * Call __sve_free() directly only if you know task can't be scheduled
 * or preempted.
 */
static void __sve_free(struct task_struct *task)
{
        kfree(task->thread.sve_state);
        task->thread.sve_state = NULL;
}

static void sve_free(struct task_struct *task)
{
        WARN_ON(test_tsk_thread_flag(task, TIF_SVE));

        __sve_free(task);
}


/* Offset of FFR in the SVE register dump */
static size_t sve_ffr_offset(int vl)
{
        return SVE_SIG_FFR_OFFSET(sve_vq_from_vl(vl)) - SVE_SIG_REGS_OFFSET;
}

static void *sve_pffr(struct task_struct *task)
{
        return (char *)task->thread.sve_state +
                sve_ffr_offset(task->thread.sve_vl);
}

static void change_cpacr(u64 val, u64 mask)
{
        u64 cpacr = read_sysreg(CPACR_EL1);
        u64 new = (cpacr & ~mask) | val;

        if (new != cpacr)
                write_sysreg(new, CPACR_EL1);
}

static void sve_user_disable(void)
{
        change_cpacr(0, CPACR_EL1_ZEN_EL0EN);
}

static void sve_user_enable(void)
{
        change_cpacr(CPACR_EL1_ZEN_EL0EN, CPACR_EL1_ZEN_EL0EN);
}

/*
 * TIF_SVE controls whether a task can use SVE without trapping while
 * in userspace, and also the way a task's FPSIMD/SVE state is stored
 * in thread_struct.
 *
 * The kernel uses this flag to track whether a user task is actively
 * using SVE, and therefore whether full SVE register state needs to
 * be tracked.  If not, the cheaper FPSIMD context handling code can
 * be used instead of the more costly SVE equivalents.
 *
 *  * TIF_SVE set:
 *
 *    The task can execute SVE instructions while in userspace without
 *    trapping to the kernel.
 *
 *    When stored, Z0-Z31 (incorporating Vn in bits[127:0] or the
 *    corresponding Zn), P0-P15 and FFR are encoded in in
 *    task->thread.sve_state, formatted appropriately for vector
 *    length task->thread.sve_vl.
 *
 *    task->thread.sve_state must point to a valid buffer at least
 *    sve_state_size(task) bytes in size.
 *
 *    During any syscall, the kernel may optionally clear TIF_SVE and
 *    discard the vector state except for the FPSIMD subset.
 *
 *  * TIF_SVE clear:
 *
 *    An attempt by the user task to execute an SVE instruction causes
 *    do_sve_acc() to be called, which does some preparation and then
 *    sets TIF_SVE.
 *
 *    When stored, FPSIMD registers V0-V31 are encoded in
 *    task->thread.uw.fpsimd_state; bits [max : 128] for each of Z0-Z31 are
 *    logically zero but not stored anywhere; P0-P15 and FFR are not
 *    stored and have unspecified values from userspace's point of
 *    view.  For hygiene purposes, the kernel zeroes them on next use,
 *    but userspace is discouraged from relying on this.
 *
 *    task->thread.sve_state does not need to be non-NULL, valid or any
 *    particular size: it must not be dereferenced.
 *
 *  * FPSR and FPCR are always stored in task->thread.uw.fpsimd_state
 *    irrespective of whether TIF_SVE is clear or set, since these are
 *    not vector length dependent.
 */

/*
 * Update current's FPSIMD/SVE registers from thread_struct.
 *
 * This function should be called only when the FPSIMD/SVE state in
 * thread_struct is known to be up to date, when preparing to enter
 * userspace.
 *
 * Softirqs (and preemption) must be disabled.
 */
static void task_fpsimd_load(void)
{
        WARN_ON(!in_softirq() && !irqs_disabled());

        if (system_supports_sve() && test_thread_flag(TIF_SVE))
                sve_load_state(sve_pffr(current),
                               &current->thread.uw.fpsimd_state.fpsr,
                               sve_vq_from_vl(current->thread.sve_vl) - 1);
        else
                fpsimd_load_state(&current->thread.uw.fpsimd_state);

        if (system_supports_sve()) {
                /* Toggle SVE trapping for userspace if needed */
                if (test_thread_flag(TIF_SVE))
                        sve_user_enable();
                else
                        sve_user_disable();

                /* Serialised by exception return to user */
        }
}

/*
 * Ensure FPSIMD/SVE storage in memory for the loaded context is up to
 * date with respect to the CPU registers.
 *
 * Softirqs (and preemption) must be disabled.
 */
static void fpsimd_save(void)
{
        struct user_fpsimd_state *st = __this_cpu_read(fpsimd_last_state.st);
        /* set by fpsimd_bind_to_cpu() */

        WARN_ON(!in_softirq() && !irqs_disabled());

        if (!test_thread_flag(TIF_FOREIGN_FPSTATE)) {
                if (system_supports_sve() && test_thread_flag(TIF_SVE)) {
                        if (WARN_ON(sve_get_vl() != current->thread.sve_vl)) {
                                /*
                                 * Can't save the user regs, so current would
                                 * re-enter user with corrupt state.
                                 * There's no way to recover, so kill it:
                                 */
                                force_signal_inject(SIGKILL, SI_KERNEL, 0);
                                return;
                        }

                        sve_save_state(sve_pffr(current), &st->fpsr);
                } else
                        fpsimd_save_state(st);
        }
}

/*
 * Helpers to translate bit indices in sve_vq_map to VQ values (and
 * vice versa).  This allows find_next_bit() to be used to find the
 * _maximum_ VQ not exceeding a certain value.
 */

static unsigned int vq_to_bit(unsigned int vq)
{
        return SVE_VQ_MAX - vq;
}

static unsigned int bit_to_vq(unsigned int bit)
{
        if (WARN_ON(bit >= SVE_VQ_MAX))
                bit = SVE_VQ_MAX - 1;

        return SVE_VQ_MAX - bit;
}

/*
 * All vector length selection from userspace comes through here.
 * We're on a slow path, so some sanity-checks are included.
 * If things go wrong there's a bug somewhere, but try to fall back to a
 * safe choice.
 */
static unsigned int find_supported_vector_length(unsigned int vl)
{
        int bit;
        int max_vl = sve_max_vl;

        if (WARN_ON(!sve_vl_valid(vl)))
                vl = SVE_VL_MIN;

        if (WARN_ON(!sve_vl_valid(max_vl)))
                max_vl = SVE_VL_MIN;

        if (vl > max_vl)
                vl = max_vl;

        bit = find_next_bit(sve_vq_map, SVE_VQ_MAX,
                            vq_to_bit(sve_vq_from_vl(vl)));
        return sve_vl_from_vq(bit_to_vq(bit));
}

#ifdef CONFIG_SYSCTL

static int sve_proc_do_default_vl(struct ctl_table *table, int write,
                                  void __user *buffer, size_t *lenp,
                                  loff_t *ppos)
{
        int ret;
        int vl = sve_default_vl;
        struct ctl_table tmp_table = {
                .data = &vl,
                .maxlen = sizeof(vl),
        };

        ret = proc_dointvec(&tmp_table, write, buffer, lenp, ppos);
        if (ret || !write)
                return ret;

        /* Writing -1 has the special meaning "set to max": */
        if (vl == -1) {
                /* Fail safe if sve_max_vl wasn't initialised */
                if (WARN_ON(!sve_vl_valid(sve_max_vl)))
                        vl = SVE_VL_MIN;
                else
                        vl = sve_max_vl;

                goto chosen;
        }

        if (!sve_vl_valid(vl))
                return -EINVAL;

        vl = find_supported_vector_length(vl);
chosen:
        sve_default_vl = vl;
        return 0;
}

static struct ctl_table sve_default_vl_table[] = {
        {
                .procname       = "sve_default_vector_length",
                .mode           = 0644,
                .proc_handler   = sve_proc_do_default_vl,
        },
        { }
};

static int __init sve_sysctl_init(void)
{
        if (system_supports_sve())
                if (!register_sysctl("abi", sve_default_vl_table))
                        return -EINVAL;

        return 0;
}

#else /* ! CONFIG_SYSCTL */
static int __init sve_sysctl_init(void) { return 0; }
#endif /* ! CONFIG_SYSCTL */

#define ZREG(sve_state, vq, n) ((char *)(sve_state) +           \
        (SVE_SIG_ZREG_OFFSET(vq, n) - SVE_SIG_REGS_OFFSET))

/*
 * Transfer the FPSIMD state in task->thread.uw.fpsimd_state to
 * task->thread.sve_state.
 *
 * Task can be a non-runnable task, or current.  In the latter case,
 * softirqs (and preemption) must be disabled.
 * task->thread.sve_state must point to at least sve_state_size(task)
 * bytes of allocated kernel memory.
 * task->thread.uw.fpsimd_state must be up to date before calling this
 * function.
 */
static void fpsimd_to_sve(struct task_struct *task)
{
        unsigned int vq;
        void *sst = task->thread.sve_state;
        struct user_fpsimd_state const *fst = &task->thread.uw.fpsimd_state;
        unsigned int i;

        if (!system_supports_sve())
                return;

        vq = sve_vq_from_vl(task->thread.sve_vl);
        for (i = 0; i < 32; ++i)
                memcpy(ZREG(sst, vq, i), &fst->vregs[i],
                       sizeof(fst->vregs[i]));
}

/*
 * Transfer the SVE state in task->thread.sve_state to
 * task->thread.uw.fpsimd_state.
 *
 * Task can be a non-runnable task, or current.  In the latter case,
 * softirqs (and preemption) must be disabled.
 * task->thread.sve_state must point to at least sve_state_size(task)
 * bytes of allocated kernel memory.
 * task->thread.sve_state must be up to date before calling this function.
 */
static void sve_to_fpsimd(struct task_struct *task)
{
        unsigned int vq;
        void const *sst = task->thread.sve_state;
        struct user_fpsimd_state *fst = &task->thread.uw.fpsimd_state;
        unsigned int i;

        if (!system_supports_sve())
                return;

        vq = sve_vq_from_vl(task->thread.sve_vl);
        for (i = 0; i < 32; ++i)
                memcpy(&fst->vregs[i], ZREG(sst, vq, i),
                       sizeof(fst->vregs[i]));
}

#ifdef CONFIG_ARM64_SVE

/*
 * Return how many bytes of memory are required to store the full SVE
 * state for task, given task's currently configured vector length.
 */
size_t sve_state_size(struct task_struct const *task)
{
        return SVE_SIG_REGS_SIZE(sve_vq_from_vl(task->thread.sve_vl));
}

/*
 * Ensure that task->thread.sve_state is allocated and sufficiently large.
 *
 * This function should be used only in preparation for replacing
 * task->thread.sve_state with new data.  The memory is always zeroed
 * here to prevent stale data from showing through: this is done in
 * the interest of testability and predictability: except in the
 * do_sve_acc() case, there is no ABI requirement to hide stale data
 * written previously be task.
 */
void sve_alloc(struct task_struct *task)
{
        if (task->thread.sve_state) {
                memset(task->thread.sve_state, 0, sve_state_size(current));
                return;
        }

        /* This is a small allocation (maximum ~8KB) and Should Not Fail. */
        task->thread.sve_state =
                kzalloc(sve_state_size(task), GFP_KERNEL);

        /*
         * If future SVE revisions can have larger vectors though,
         * this may cease to be true:
         */
        BUG_ON(!task->thread.sve_state);
}


/*
 * Ensure that task->thread.sve_state is up to date with respect to
 * the user task, irrespective of when SVE is in use or not.
 *
 * This should only be called by ptrace.  task must be non-runnable.
 * task->thread.sve_state must point to at least sve_state_size(task)
 * bytes of allocated kernel memory.
 */
void fpsimd_sync_to_sve(struct task_struct *task)
{
        if (!test_tsk_thread_flag(task, TIF_SVE))
                fpsimd_to_sve(task);
}

/*
 * Ensure that task->thread.uw.fpsimd_state is up to date with respect to
 * the user task, irrespective of whether SVE is in use or not.
 *
 * This should only be called by ptrace.  task must be non-runnable.
 * task->thread.sve_state must point to at least sve_state_size(task)
 * bytes of allocated kernel memory.
 */
void sve_sync_to_fpsimd(struct task_struct *task)
{
        if (test_tsk_thread_flag(task, TIF_SVE))
                sve_to_fpsimd(task);
}

/*
 * Ensure that task->thread.sve_state is up to date with respect to
 * the task->thread.uw.fpsimd_state.
 *
 * This should only be called by ptrace to merge new FPSIMD register
 * values into a task for which SVE is currently active.
 * task must be non-runnable.
 * task->thread.sve_state must point to at least sve_state_size(task)
 * bytes of allocated kernel memory.
 * task->thread.uw.fpsimd_state must already have been initialised with
 * the new FPSIMD register values to be merged in.
 */
void sve_sync_from_fpsimd_zeropad(struct task_struct *task)
{
        unsigned int vq;
        void *sst = task->thread.sve_state;
        struct user_fpsimd_state const *fst = &task->thread.uw.fpsimd_state;
        unsigned int i;

        if (!test_tsk_thread_flag(task, TIF_SVE))
                return;

        vq = sve_vq_from_vl(task->thread.sve_vl);

        memset(sst, 0, SVE_SIG_REGS_SIZE(vq));

        for (i = 0; i < 32; ++i)
                memcpy(ZREG(sst, vq, i), &fst->vregs[i],
                       sizeof(fst->vregs[i]));
}

int sve_set_vector_length(struct task_struct *task,
                          unsigned long vl, unsigned long flags)
{
        if (flags & ~(unsigned long)(PR_SVE_VL_INHERIT |
                                     PR_SVE_SET_VL_ONEXEC))
                return -EINVAL;

        if (!sve_vl_valid(vl))
                return -EINVAL;

        /*
         * Clamp to the maximum vector length that VL-agnostic SVE code can
         * work with.  A flag may be assigned in the future to allow setting
         * of larger vector lengths without confusing older software.
         */
        if (vl > SVE_VL_ARCH_MAX)
                vl = SVE_VL_ARCH_MAX;

        vl = find_supported_vector_length(vl);

        if (flags & (PR_SVE_VL_INHERIT |
                     PR_SVE_SET_VL_ONEXEC))
                task->thread.sve_vl_onexec = vl;
        else
                /* Reset VL to system default on next exec: */
                task->thread.sve_vl_onexec = 0;

        /* Only actually set the VL if not deferred: */
        if (flags & PR_SVE_SET_VL_ONEXEC)
                goto out;

        if (vl == task->thread.sve_vl)
                goto out;

        /*
         * To ensure the FPSIMD bits of the SVE vector registers are preserved,
         * write any live register state back to task_struct, and convert to a
         * non-SVE thread.
         */
        if (task == current) {
                local_bh_disable();

                fpsimd_save();
                set_thread_flag(TIF_FOREIGN_FPSTATE);
        }

        fpsimd_flush_task_state(task);
        if (test_and_clear_tsk_thread_flag(task, TIF_SVE))
                sve_to_fpsimd(task);

        if (task == current)
                local_bh_enable();

        /*
         * Force reallocation of task SVE state to the correct size
         * on next use:
         */
        sve_free(task);

        task->thread.sve_vl = vl;

out:
        update_tsk_thread_flag(task, TIF_SVE_VL_INHERIT,
                               flags & PR_SVE_VL_INHERIT);

        return 0;
}

/*
 * Encode the current vector length and flags for return.
 * This is only required for prctl(): ptrace has separate fields
 *
 * flags are as for sve_set_vector_length().
 */
static int sve_prctl_status(unsigned long flags)
{
        int ret;

        if (flags & PR_SVE_SET_VL_ONEXEC)
                ret = current->thread.sve_vl_onexec;
        else
                ret = current->thread.sve_vl;

        if (test_thread_flag(TIF_SVE_VL_INHERIT))
                ret |= PR_SVE_VL_INHERIT;

        return ret;
}

/* PR_SVE_SET_VL */
int sve_set_current_vl(unsigned long arg)
{
        unsigned long vl, flags;
        int ret;

        vl = arg & PR_SVE_VL_LEN_MASK;
        flags = arg & ~vl;

        if (!system_supports_sve())
                return -EINVAL;

        ret = sve_set_vector_length(current, vl, flags);
        if (ret)
                return ret;

        return sve_prctl_status(flags);
}

/* PR_SVE_GET_VL */
int sve_get_current_vl(void)
{
        if (!system_supports_sve())
                return -EINVAL;

        return sve_prctl_status(0);
}

/*
 * Bitmap for temporary storage of the per-CPU set of supported vector lengths
 * during secondary boot.
 */
static DECLARE_BITMAP(sve_secondary_vq_map, SVE_VQ_MAX);

static void sve_probe_vqs(DECLARE_BITMAP(map, SVE_VQ_MAX))
{
        unsigned int vq, vl;
        unsigned long zcr;

        bitmap_zero(map, SVE_VQ_MAX);

        zcr = ZCR_ELx_LEN_MASK;
        zcr = read_sysreg_s(SYS_ZCR_EL1) & ~zcr;

        for (vq = SVE_VQ_MAX; vq >= SVE_VQ_MIN; --vq) {
                write_sysreg_s(zcr | (vq - 1), SYS_ZCR_EL1); /* self-syncing */
                vl = sve_get_vl();
                vq = sve_vq_from_vl(vl); /* skip intervening lengths */
                set_bit(vq_to_bit(vq), map);
        }
}

void __init sve_init_vq_map(void)
{
        sve_probe_vqs(sve_vq_map);
}

/*
 * If we haven't committed to the set of supported VQs yet, filter out
 * those not supported by the current CPU.
 */
void sve_update_vq_map(void)
{
        sve_probe_vqs(sve_secondary_vq_map);
        bitmap_and(sve_vq_map, sve_vq_map, sve_secondary_vq_map, SVE_VQ_MAX);
}

/* Check whether the current CPU supports all VQs in the committed set */
int sve_verify_vq_map(void)
{
        int ret = 0;

        sve_probe_vqs(sve_secondary_vq_map);
        bitmap_andnot(sve_secondary_vq_map, sve_vq_map, sve_secondary_vq_map,
                      SVE_VQ_MAX);
        if (!bitmap_empty(sve_secondary_vq_map, SVE_VQ_MAX)) {
                pr_warn("SVE: cpu%d: Required vector length(s) missing\n",
                        smp_processor_id());
                ret = -EINVAL;
        }

        return ret;
}

static void __init sve_efi_setup(void)
{
        if (!IS_ENABLED(CONFIG_EFI))
                return;

        /*
         * alloc_percpu() warns and prints a backtrace if this goes wrong.
         * This is evidence of a crippled system and we are returning void,
         * so no attempt is made to handle this situation here.
         */
        if (!sve_vl_valid(sve_max_vl))
                goto fail;

        efi_sve_state = __alloc_percpu(
                SVE_SIG_REGS_SIZE(sve_vq_from_vl(sve_max_vl)), SVE_VQ_BYTES);
        if (!efi_sve_state)
                goto fail;

        return;

fail:
        panic("Cannot allocate percpu memory for EFI SVE save/restore");
}

/*
 * Enable SVE for EL1.
 * Intended for use by the cpufeatures code during CPU boot.
 */
void sve_kernel_enable(const struct arm64_cpu_capabilities *__always_unused p)
{
        write_sysreg(read_sysreg(CPACR_EL1) | CPACR_EL1_ZEN_EL1EN, CPACR_EL1);
        isb();
}

void __init sve_setup(void)
{
        u64 zcr;

        if (!system_supports_sve())
                return;

        /*
         * The SVE architecture mandates support for 128-bit vectors,
         * so sve_vq_map must have at least SVE_VQ_MIN set.
         * If something went wrong, at least try to patch it up:
         */
        if (WARN_ON(!test_bit(vq_to_bit(SVE_VQ_MIN), sve_vq_map)))
                set_bit(vq_to_bit(SVE_VQ_MIN), sve_vq_map);

        zcr = read_sanitised_ftr_reg(SYS_ZCR_EL1);
        sve_max_vl = sve_vl_from_vq((zcr & ZCR_ELx_LEN_MASK) + 1);

        /*
         * Sanity-check that the max VL we determined through CPU features
         * corresponds properly to sve_vq_map.  If not, do our best:
         */
        if (WARN_ON(sve_max_vl != find_supported_vector_length(sve_max_vl)))
                sve_max_vl = find_supported_vector_length(sve_max_vl);

        /*
         * For the default VL, pick the maximum supported value <= 64.
         * VL == 64 is guaranteed not to grow the signal frame.
         */
        sve_default_vl = find_supported_vector_length(64);

        pr_info("SVE: maximum available vector length %u bytes per vector\n",
                sve_max_vl);
        pr_info("SVE: default vector length %u bytes per vector\n",
                sve_default_vl);

        sve_efi_setup();
}

/*
 * Called from the put_task_struct() path, which cannot get here
 * unless dead_task is really dead and not schedulable.
 */
void fpsimd_release_task(struct task_struct *dead_task)
{
        __sve_free(dead_task);
}

#endif /* CONFIG_ARM64_SVE */

/*
 * Trapped SVE access
 *
 * Storage is allocated for the full SVE state, the current FPSIMD
 * register contents are migrated across, and TIF_SVE is set so that
 * the SVE access trap will be disabled the next time this task
 * reaches ret_to_user.
 *
 * TIF_SVE should be clear on entry: otherwise, task_fpsimd_load()
 * would have disabled the SVE access trap for userspace during
 * ret_to_user, making an SVE access trap impossible in that case.
 */
asmlinkage void do_sve_acc(unsigned int esr, struct pt_regs *regs)
{
        /* Even if we chose not to use SVE, the hardware could still trap: */
        if (unlikely(!system_supports_sve()) || WARN_ON(is_compat_task())) {
                force_signal_inject(SIGILL, ILL_ILLOPC, regs->pc);
                return;
        }

        sve_alloc(current);

        local_bh_disable();

        fpsimd_save();
        fpsimd_to_sve(current);

        /* Force ret_to_user to reload the registers: */
        fpsimd_flush_task_state(current);
        set_thread_flag(TIF_FOREIGN_FPSTATE);

        if (test_and_set_thread_flag(TIF_SVE))
                WARN_ON(1); /* SVE access shouldn't have trapped */

        local_bh_enable();
}

/*
 * Trapped FP/ASIMD access.
 */
asmlinkage void do_fpsimd_acc(unsigned int esr, struct pt_regs *regs)
{
        /* TODO: implement lazy context saving/restoring */
        WARN_ON(1);
}

/*
 * Raise a SIGFPE for the current process.
 */
asmlinkage void do_fpsimd_exc(unsigned int esr, struct pt_regs *regs)
{
        siginfo_t info;
        unsigned int si_code = FPE_FLTUNK;

        if (esr & ESR_ELx_FP_EXC_TFV) {
                if (esr & FPEXC_IOF)
                        si_code = FPE_FLTINV;
                else if (esr & FPEXC_DZF)
                        si_code = FPE_FLTDIV;
                else if (esr & FPEXC_OFF)
                        si_code = FPE_FLTOVF;
                else if (esr & FPEXC_UFF)
                        si_code = FPE_FLTUND;
                else if (esr & FPEXC_IXF)
                        si_code = FPE_FLTRES;
        }

        memset(&info, 0, sizeof(info));
        info.si_signo = SIGFPE;
        info.si_code = si_code;
        info.si_addr = (void __user *)instruction_pointer(regs);

        send_sig_info(SIGFPE, &info, current);
}

void fpsimd_thread_switch(struct task_struct *next)
{
        if (!system_supports_fpsimd())
                return;
        /*
         * Save the current FPSIMD state to memory, but only if whatever is in
         * the registers is in fact the most recent userland FPSIMD state of
         * 'current'.
         */
        if (current->mm)
                fpsimd_save();

        if (next->mm) {
                /*
                 * If we are switching to a task whose most recent userland
                 * FPSIMD state is already in the registers of *this* cpu,
                 * we can skip loading the state from memory. Otherwise, set
                 * the TIF_FOREIGN_FPSTATE flag so the state will be loaded
                 * upon the next return to userland.
                 */
                bool wrong_task = __this_cpu_read(fpsimd_last_state.st) !=
                                        &next->thread.uw.fpsimd_state;
                bool wrong_cpu = next->thread.fpsimd_cpu != smp_processor_id();

                update_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE,
                                       wrong_task || wrong_cpu);
        }
}

void fpsimd_flush_thread(void)
{
        int vl, supported_vl;

        if (!system_supports_fpsimd())
                return;

        local_bh_disable();

        memset(&current->thread.uw.fpsimd_state, 0,
               sizeof(current->thread.uw.fpsimd_state));
        fpsimd_flush_task_state(current);

        if (system_supports_sve()) {
                clear_thread_flag(TIF_SVE);
                sve_free(current);

                /*
                 * Reset the task vector length as required.
                 * This is where we ensure that all user tasks have a valid
                 * vector length configured: no kernel task can become a user
                 * task without an exec and hence a call to this function.
                 * By the time the first call to this function is made, all
                 * early hardware probing is complete, so sve_default_vl
                 * should be valid.
                 * If a bug causes this to go wrong, we make some noise and
                 * try to fudge thread.sve_vl to a safe value here.
                 */
                vl = current->thread.sve_vl_onexec ?
                        current->thread.sve_vl_onexec : sve_default_vl;

                if (WARN_ON(!sve_vl_valid(vl)))
                        vl = SVE_VL_MIN;

                supported_vl = find_supported_vector_length(vl);
                if (WARN_ON(supported_vl != vl))
                        vl = supported_vl;

                current->thread.sve_vl = vl;

                /*
                 * If the task is not set to inherit, ensure that the vector
                 * length will be reset by a subsequent exec:
                 */
                if (!test_thread_flag(TIF_SVE_VL_INHERIT))
                        current->thread.sve_vl_onexec = 0;
        }

        set_thread_flag(TIF_FOREIGN_FPSTATE);

        local_bh_enable();
}

/*
 * Save the userland FPSIMD state of 'current' to memory, but only if the state
 * currently held in the registers does in fact belong to 'current'
 */
void fpsimd_preserve_current_state(void)
{
        if (!system_supports_fpsimd())
                return;

        local_bh_disable();
        fpsimd_save();
        local_bh_enable();
}

/*
 * Like fpsimd_preserve_current_state(), but ensure that
 * current->thread.uw.fpsimd_state is updated so that it can be copied to
 * the signal frame.
 */
void fpsimd_signal_preserve_current_state(void)
{
        fpsimd_preserve_current_state();
        if (system_supports_sve() && test_thread_flag(TIF_SVE))
                sve_to_fpsimd(current);
}

/*
 * Associate current's FPSIMD context with this cpu
 * Preemption must be disabled when calling this function.
 */
static void fpsimd_bind_to_cpu(void)
{
        struct fpsimd_last_state_struct *last =
                this_cpu_ptr(&fpsimd_last_state);

        last->st = &current->thread.uw.fpsimd_state;
        last->sve_in_use = test_thread_flag(TIF_SVE);
        current->thread.fpsimd_cpu = smp_processor_id();
}

/*
 * Load the userland FPSIMD state of 'current' from memory, but only if the
 * FPSIMD state already held in the registers is /not/ the most recent FPSIMD
 * state of 'current'
 */
void fpsimd_restore_current_state(void)
{
        if (!system_supports_fpsimd())
                return;

        local_bh_disable();

        if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE)) {
                task_fpsimd_load();
                fpsimd_bind_to_cpu();
        }

        local_bh_enable();
}

/*
 * Load an updated userland FPSIMD state for 'current' from memory and set the
 * flag that indicates that the FPSIMD register contents are the most recent
 * FPSIMD state of 'current'
 */
void fpsimd_update_current_state(struct user_fpsimd_state const *state)
{
        if (!system_supports_fpsimd())
                return;

        local_bh_disable();

        current->thread.uw.fpsimd_state = *state;
        if (system_supports_sve() && test_thread_flag(TIF_SVE))
                fpsimd_to_sve(current);

        task_fpsimd_load();

        if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE))
                fpsimd_bind_to_cpu();

        local_bh_enable();
}

/*
 * Invalidate live CPU copies of task t's FPSIMD state
 */
void fpsimd_flush_task_state(struct task_struct *t)
{
        t->thread.fpsimd_cpu = NR_CPUS;
}

static inline void fpsimd_flush_cpu_state(void)
{
        __this_cpu_write(fpsimd_last_state.st, NULL);
        set_thread_flag(TIF_FOREIGN_FPSTATE);
}

/*
 * Invalidate any task SVE state currently held in this CPU's regs.
 *
 * This is used to prevent the kernel from trying to reuse SVE register data
 * that is detroyed by KVM guest enter/exit.  This function should go away when
 * KVM SVE support is implemented.  Don't use it for anything else.
 */
#ifdef CONFIG_ARM64_SVE
void sve_flush_cpu_state(void)
{
        struct fpsimd_last_state_struct const *last =
                this_cpu_ptr(&fpsimd_last_state);

        if (last->st && last->sve_in_use)
                fpsimd_flush_cpu_state();
}
#endif /* CONFIG_ARM64_SVE */

#ifdef CONFIG_KERNEL_MODE_NEON

DEFINE_PER_CPU(bool, kernel_neon_busy);
EXPORT_PER_CPU_SYMBOL(kernel_neon_busy);

/*
 * Kernel-side NEON support functions
 */

/*
 * kernel_neon_begin(): obtain the CPU FPSIMD registers for use by the calling
 * context
 *
 * Must not be called unless may_use_simd() returns true.
 * Task context in the FPSIMD registers is saved back to memory as necessary.
 *
 * A matching call to kernel_neon_end() must be made before returning from the
 * calling context.
 *
 * The caller may freely use the FPSIMD registers until kernel_neon_end() is
 * called.
 */
void kernel_neon_begin(void)
{
        if (WARN_ON(!system_supports_fpsimd()))
                return;

        BUG_ON(!may_use_simd());

        local_bh_disable();

        __this_cpu_write(kernel_neon_busy, true);

        /* Save unsaved task fpsimd state, if any: */
        if (current->mm)
                fpsimd_save();

        /* Invalidate any task state remaining in the fpsimd regs: */
        fpsimd_flush_cpu_state();

        preempt_disable();

        local_bh_enable();
}
EXPORT_SYMBOL(kernel_neon_begin);

/*
 * kernel_neon_end(): give the CPU FPSIMD registers back to the current task
 *
 * Must be called from a context in which kernel_neon_begin() was previously
 * called, with no call to kernel_neon_end() in the meantime.
 *
 * The caller must not use the FPSIMD registers after this function is called,
 * unless kernel_neon_begin() is called again in the meantime.
 */
void kernel_neon_end(void)
{
        bool busy;

        if (!system_supports_fpsimd())
                return;

        busy = __this_cpu_xchg(kernel_neon_busy, false);
        WARN_ON(!busy); /* No matching kernel_neon_begin()? */

        preempt_enable();
}
EXPORT_SYMBOL(kernel_neon_end);

#ifdef CONFIG_EFI

static DEFINE_PER_CPU(struct user_fpsimd_state, efi_fpsimd_state);
static DEFINE_PER_CPU(bool, efi_fpsimd_state_used);
static DEFINE_PER_CPU(bool, efi_sve_state_used);

/*
 * EFI runtime services support functions
 *
 * The ABI for EFI runtime services allows EFI to use FPSIMD during the call.
 * This means that for EFI (and only for EFI), we have to assume that FPSIMD
 * is always used rather than being an optional accelerator.
 *
 * These functions provide the necessary support for ensuring FPSIMD
 * save/restore in the contexts from which EFI is used.
 *
 * Do not use them for any other purpose -- if tempted to do so, you are
 * either doing something wrong or you need to propose some refactoring.
 */

/*
 * __efi_fpsimd_begin(): prepare FPSIMD for making an EFI runtime services call
 */
void __efi_fpsimd_begin(void)
{
        if (!system_supports_fpsimd())
                return;

        WARN_ON(preemptible());

        if (may_use_simd()) {
                kernel_neon_begin();
        } else {
                /*
                 * If !efi_sve_state, SVE can't be in use yet and doesn't need
                 * preserving:
                 */
                if (system_supports_sve() && likely(efi_sve_state)) {
                        char *sve_state = this_cpu_ptr(efi_sve_state);

                        __this_cpu_write(efi_sve_state_used, true);

                        sve_save_state(sve_state + sve_ffr_offset(sve_max_vl),
                                       &this_cpu_ptr(&efi_fpsimd_state)->fpsr);
                } else {
                        fpsimd_save_state(this_cpu_ptr(&efi_fpsimd_state));
                }

                __this_cpu_write(efi_fpsimd_state_used, true);
        }
}

/*
 * __efi_fpsimd_end(): clean up FPSIMD after an EFI runtime services call
 */
void __efi_fpsimd_end(void)
{
        if (!system_supports_fpsimd())
                return;

        if (!__this_cpu_xchg(efi_fpsimd_state_used, false)) {
                kernel_neon_end();
        } else {
                if (system_supports_sve() &&
                    likely(__this_cpu_read(efi_sve_state_used))) {
                        char const *sve_state = this_cpu_ptr(efi_sve_state);

                        sve_load_state(sve_state + sve_ffr_offset(sve_max_vl),
                                       &this_cpu_ptr(&efi_fpsimd_state)->fpsr,
                                       sve_vq_from_vl(sve_get_vl()) - 1);

                        __this_cpu_write(efi_sve_state_used, false);
                } else {
                        fpsimd_load_state(this_cpu_ptr(&efi_fpsimd_state));
                }
        }
}

#endif /* CONFIG_EFI */

#endif /* CONFIG_KERNEL_MODE_NEON */

#ifdef CONFIG_CPU_PM
static int fpsimd_cpu_pm_notifier(struct notifier_block *self,
                                  unsigned long cmd, void *v)
{
        switch (cmd) {
        case CPU_PM_ENTER:
                if (current->mm)
                        fpsimd_save();
                fpsimd_flush_cpu_state();
                break;
        case CPU_PM_EXIT:
                break;
        case CPU_PM_ENTER_FAILED:
        default:
                return NOTIFY_DONE;
        }
        return NOTIFY_OK;
}

static struct notifier_block fpsimd_cpu_pm_notifier_block = {
        .notifier_call = fpsimd_cpu_pm_notifier,
};

static void __init fpsimd_pm_init(void)
{
        cpu_pm_register_notifier(&fpsimd_cpu_pm_notifier_block);
}

#else
static inline void fpsimd_pm_init(void) { }
#endif /* CONFIG_CPU_PM */

#ifdef CONFIG_HOTPLUG_CPU
static int fpsimd_cpu_dead(unsigned int cpu)
{
        per_cpu(fpsimd_last_state.st, cpu) = NULL;
        return 0;
}

static inline void fpsimd_hotplug_init(void)
{
        cpuhp_setup_state_nocalls(CPUHP_ARM64_FPSIMD_DEAD, "arm64/fpsimd:dead",
                                  NULL, fpsimd_cpu_dead);
}

#else
static inline void fpsimd_hotplug_init(void) { }
#endif

/*
 * FP/SIMD support code initialisation.
 */
static int __init fpsimd_init(void)
{
        if (elf_hwcap & HWCAP_FP) {
                fpsimd_pm_init();
                fpsimd_hotplug_init();
        } else {
                pr_notice("Floating-point is not implemented\n");
        }

        if (!(elf_hwcap & HWCAP_ASIMD))
                pr_notice("Advanced SIMD is not implemented\n");

        return sve_sysctl_init();
}
core_initcall(fpsimd_init);