[thirdparty/kernel/linux.git] / virt / kvm / pfncache.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * Kernel-based Virtual Machine driver for Linux
 *
 * This module enables kernel and guest-mode vCPU access to guest physical
 * memory with suitable invalidation mechanisms.
 *
 * Copyright © 2021 Amazon.com, Inc. or its affiliates.
 *
 * Authors:
 *   David Woodhouse <dwmw2@infradead.org>
 */

#include <linux/kvm_host.h>
#include <linux/kvm.h>
#include <linux/highmem.h>
#include <linux/module.h>
#include <linux/errno.h>

#include "kvm_mm.h"

/*
 * MMU notifier 'invalidate_range_start' hook.
 */
void gfn_to_pfn_cache_invalidate_start(struct kvm *kvm, unsigned long start,
                                       unsigned long end, bool may_block)
{
        struct gfn_to_pfn_cache *gpc;

        spin_lock(&kvm->gpc_lock);
        list_for_each_entry(gpc, &kvm->gpc_list, list) {
                read_lock_irq(&gpc->lock);

                /* Only a single page so no need to care about length */
                if (gpc->valid && !is_error_noslot_pfn(gpc->pfn) &&
                    gpc->uhva >= start && gpc->uhva < end) {
                        read_unlock_irq(&gpc->lock);

                        /*
                         * There is a small window here where the cache could
                         * be modified, and invalidation would no longer be
                         * necessary. Hence check again whether invalidation
                         * is still necessary once the write lock has been
                         * acquired.
                         */

                        write_lock_irq(&gpc->lock);
                        if (gpc->valid && !is_error_noslot_pfn(gpc->pfn) &&
                            gpc->uhva >= start && gpc->uhva < end)
                                gpc->valid = false;
                        write_unlock_irq(&gpc->lock);
                        continue;
                }

                read_unlock_irq(&gpc->lock);
        }
        spin_unlock(&kvm->gpc_lock);
}

static bool kvm_gpc_is_valid_len(gpa_t gpa, unsigned long uhva,
                                 unsigned long len)
{
        unsigned long offset = kvm_is_error_gpa(gpa) ? offset_in_page(uhva) :
                                                       offset_in_page(gpa);

        /*
         * The cached access must fit within a single page. The 'len' argument
         * to activate() and refresh() exists only to enforce that.
         */
        return offset + len <= PAGE_SIZE;
}

bool kvm_gpc_check(struct gfn_to_pfn_cache *gpc, unsigned long len)
{
        struct kvm_memslots *slots = kvm_memslots(gpc->kvm);

        if (!gpc->active)
                return false;

        /*
         * If the page was cached from a memslot, make sure the memslots have
         * not been re-configured.
         */
        if (!kvm_is_error_gpa(gpc->gpa) && gpc->generation != slots->generation)
                return false;

        if (kvm_is_error_hva(gpc->uhva))
                return false;

        if (!kvm_gpc_is_valid_len(gpc->gpa, gpc->uhva, len))
                return false;

        if (!gpc->valid)
                return false;

        return true;
}

static void *gpc_map(kvm_pfn_t pfn)
{
        if (pfn_valid(pfn))
                return kmap(pfn_to_page(pfn));

#ifdef CONFIG_HAS_IOMEM
        return memremap(pfn_to_hpa(pfn), PAGE_SIZE, MEMREMAP_WB);
#else
        return NULL;
#endif
}

static void gpc_unmap(kvm_pfn_t pfn, void *khva)
{
        /* Unmap the old pfn/page if it was mapped before. */
        if (is_error_noslot_pfn(pfn) || !khva)
                return;

        if (pfn_valid(pfn)) {
                kunmap(pfn_to_page(pfn));
                return;
        }

#ifdef CONFIG_HAS_IOMEM
        memunmap(khva);
#endif
}

static inline bool mmu_notifier_retry_cache(struct kvm *kvm, unsigned long mmu_seq)
{
        /*
         * mn_active_invalidate_count acts for all intents and purposes
         * like mmu_invalidate_in_progress here; but the latter cannot
         * be used here because the invalidation of caches in the
         * mmu_notifier event occurs _before_ mmu_invalidate_in_progress
         * is elevated.
         *
         * Note, it does not matter that mn_active_invalidate_count
         * is not protected by gpc->lock.  It is guaranteed to
         * be elevated before the mmu_notifier acquires gpc->lock, and
         * isn't dropped until after mmu_invalidate_seq is updated.
         */
        if (kvm->mn_active_invalidate_count)
                return true;

        /*
         * Ensure mn_active_invalidate_count is read before
         * mmu_invalidate_seq.  This pairs with the smp_wmb() in
         * mmu_notifier_invalidate_range_end() to guarantee either the
         * old (non-zero) value of mn_active_invalidate_count or the
         * new (incremented) value of mmu_invalidate_seq is observed.
         */
        smp_rmb();
        return kvm->mmu_invalidate_seq != mmu_seq;
}

static kvm_pfn_t hva_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
{
        /* Note, the new page offset may be different than the old! */
        void *old_khva = (void *)PAGE_ALIGN_DOWN((uintptr_t)gpc->khva);
        kvm_pfn_t new_pfn = KVM_PFN_ERR_FAULT;
        void *new_khva = NULL;
        unsigned long mmu_seq;

        lockdep_assert_held(&gpc->refresh_lock);

        lockdep_assert_held_write(&gpc->lock);

        /*
         * Invalidate the cache prior to dropping gpc->lock, the gpa=>uhva
         * assets have already been updated and so a concurrent check() from a
         * different task may not fail the gpa/uhva/generation checks.
         */
        gpc->valid = false;

        do {
                mmu_seq = gpc->kvm->mmu_invalidate_seq;
                smp_rmb();

                write_unlock_irq(&gpc->lock);

                /*
                 * If the previous iteration "failed" due to an mmu_notifier
                 * event, release the pfn and unmap the kernel virtual address
                 * from the previous attempt.  Unmapping might sleep, so this
                 * needs to be done after dropping the lock.  Opportunistically
                 * check for resched while the lock isn't held.
                 */
                if (new_pfn != KVM_PFN_ERR_FAULT) {
                        /*
                         * Keep the mapping if the previous iteration reused
                         * the existing mapping and didn't create a new one.
                         */
                        if (new_khva != old_khva)
                                gpc_unmap(new_pfn, new_khva);

                        kvm_release_pfn_clean(new_pfn);

                        cond_resched();
                }

                /* We always request a writeable mapping */
                new_pfn = hva_to_pfn(gpc->uhva, false, false, NULL, true, NULL);
                if (is_error_noslot_pfn(new_pfn))
                        goto out_error;

                /*
                 * Obtain a new kernel mapping if KVM itself will access the
                 * pfn.  Note, kmap() and memremap() can both sleep, so this
                 * too must be done outside of gpc->lock!
                 */
                if (new_pfn == gpc->pfn)
                        new_khva = old_khva;
                else
                        new_khva = gpc_map(new_pfn);

                if (!new_khva) {
                        kvm_release_pfn_clean(new_pfn);
                        goto out_error;
                }

                write_lock_irq(&gpc->lock);

                /*
                 * Other tasks must wait for _this_ refresh to complete before
                 * attempting to refresh.
                 */
                WARN_ON_ONCE(gpc->valid);
        } while (mmu_notifier_retry_cache(gpc->kvm, mmu_seq));

        gpc->valid = true;
        gpc->pfn = new_pfn;
        gpc->khva = new_khva + offset_in_page(gpc->uhva);

        /*
         * Put the reference to the _new_ pfn.  The pfn is now tracked by the
         * cache and can be safely migrated, swapped, etc... as the cache will
         * invalidate any mappings in response to relevant mmu_notifier events.
         */
        kvm_release_pfn_clean(new_pfn);

        return 0;

out_error:
        write_lock_irq(&gpc->lock);

        return -EFAULT;
}

static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long uhva,
                             unsigned long len)
{
        unsigned long page_offset;
        bool unmap_old = false;
        unsigned long old_uhva;
        kvm_pfn_t old_pfn;
        bool hva_change = false;
        void *old_khva;
        int ret;

        /* Either gpa or uhva must be valid, but not both */
        if (WARN_ON_ONCE(kvm_is_error_gpa(gpa) == kvm_is_error_hva(uhva)))
                return -EINVAL;

        if (!kvm_gpc_is_valid_len(gpa, uhva, len))
                return -EINVAL;

        lockdep_assert_held(&gpc->refresh_lock);

        write_lock_irq(&gpc->lock);

        if (!gpc->active) {
                ret = -EINVAL;
                goto out_unlock;
        }

        old_pfn = gpc->pfn;
        old_khva = (void *)PAGE_ALIGN_DOWN((uintptr_t)gpc->khva);
        old_uhva = PAGE_ALIGN_DOWN(gpc->uhva);

        if (kvm_is_error_gpa(gpa)) {
                page_offset = offset_in_page(uhva);

                gpc->gpa = INVALID_GPA;
                gpc->memslot = NULL;
                gpc->uhva = PAGE_ALIGN_DOWN(uhva);

                if (gpc->uhva != old_uhva)
                        hva_change = true;
        } else {
                struct kvm_memslots *slots = kvm_memslots(gpc->kvm);

                page_offset = offset_in_page(gpa);

                if (gpc->gpa != gpa || gpc->generation != slots->generation ||
                    kvm_is_error_hva(gpc->uhva)) {
                        gfn_t gfn = gpa_to_gfn(gpa);

                        gpc->gpa = gpa;
                        gpc->generation = slots->generation;
                        gpc->memslot = __gfn_to_memslot(slots, gfn);
                        gpc->uhva = gfn_to_hva_memslot(gpc->memslot, gfn);

                        if (kvm_is_error_hva(gpc->uhva)) {
                                ret = -EFAULT;
                                goto out;
                        }

                        /*
                         * Even if the GPA and/or the memslot generation changed, the
                         * HVA may still be the same.
                         */
                        if (gpc->uhva != old_uhva)
                                hva_change = true;
                } else {
                        gpc->uhva = old_uhva;
                }
        }

        /* Note: the offset must be correct before calling hva_to_pfn_retry() */
        gpc->uhva += page_offset;

        /*
         * If the userspace HVA changed or the PFN was already invalid,
         * drop the lock and do the HVA to PFN lookup again.
         */
        if (!gpc->valid || hva_change) {
                ret = hva_to_pfn_retry(gpc);
        } else {
                /*
                 * If the HVA→PFN mapping was already valid, don't unmap it.
                 * But do update gpc->khva because the offset within the page
                 * may have changed.
                 */
                gpc->khva = old_khva + page_offset;
                ret = 0;
                goto out_unlock;
        }

 out:
        /*
         * Invalidate the cache and purge the pfn/khva if the refresh failed.
         * Some/all of the uhva, gpa, and memslot generation info may still be
         * valid, leave it as is.
         */
        if (ret) {
                gpc->valid = false;
                gpc->pfn = KVM_PFN_ERR_FAULT;
                gpc->khva = NULL;
        }

        /* Detect a pfn change before dropping the lock! */
        unmap_old = (old_pfn != gpc->pfn);

out_unlock:
        write_unlock_irq(&gpc->lock);

        if (unmap_old)
                gpc_unmap(old_pfn, old_khva);

        return ret;
}

int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, unsigned long len)
{
        unsigned long uhva;

        guard(mutex)(&gpc->refresh_lock);

        /*
         * If the GPA is valid then ignore the HVA, as a cache can be GPA-based
         * or HVA-based, not both.  For GPA-based caches, the HVA will be
         * recomputed during refresh if necessary.
         */
        uhva = kvm_is_error_gpa(gpc->gpa) ? gpc->uhva : KVM_HVA_ERR_BAD;

        return __kvm_gpc_refresh(gpc, gpc->gpa, uhva, len);
}

void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm)
{
        rwlock_init(&gpc->lock);
        mutex_init(&gpc->refresh_lock);

        gpc->kvm = kvm;
        gpc->pfn = KVM_PFN_ERR_FAULT;
        gpc->gpa = INVALID_GPA;
        gpc->uhva = KVM_HVA_ERR_BAD;
        gpc->active = gpc->valid = false;
}

static int __kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long uhva,
                              unsigned long len)
{
        struct kvm *kvm = gpc->kvm;

        guard(mutex)(&gpc->refresh_lock);

        if (!gpc->active) {
                if (KVM_BUG_ON(gpc->valid, kvm))
                        return -EIO;

                spin_lock(&kvm->gpc_lock);
                list_add(&gpc->list, &kvm->gpc_list);
                spin_unlock(&kvm->gpc_lock);

                /*
                 * Activate the cache after adding it to the list, a concurrent
                 * refresh must not establish a mapping until the cache is
                 * reachable by mmu_notifier events.
                 */
                write_lock_irq(&gpc->lock);
                gpc->active = true;
                write_unlock_irq(&gpc->lock);
        }
        return __kvm_gpc_refresh(gpc, gpa, uhva, len);
}

int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len)
{
        return __kvm_gpc_activate(gpc, gpa, KVM_HVA_ERR_BAD, len);
}

int kvm_gpc_activate_hva(struct gfn_to_pfn_cache *gpc, unsigned long uhva, unsigned long len)
{
        return __kvm_gpc_activate(gpc, INVALID_GPA, uhva, len);
}

void kvm_gpc_deactivate(struct gfn_to_pfn_cache *gpc)
{
        struct kvm *kvm = gpc->kvm;
        kvm_pfn_t old_pfn;
        void *old_khva;

        guard(mutex)(&gpc->refresh_lock);

        if (gpc->active) {
                /*
                 * Deactivate the cache before removing it from the list, KVM
                 * must stall mmu_notifier events until all users go away, i.e.
                 * until gpc->lock is dropped and refresh is guaranteed to fail.
                 */
                write_lock_irq(&gpc->lock);
                gpc->active = false;
                gpc->valid = false;

                /*
                 * Leave the GPA => uHVA cache intact, it's protected by the
                 * memslot generation.  The PFN lookup needs to be redone every
                 * time as mmu_notifier protection is lost when the cache is
                 * removed from the VM's gpc_list.
                 */
                old_khva = gpc->khva - offset_in_page(gpc->khva);
                gpc->khva = NULL;

                old_pfn = gpc->pfn;
                gpc->pfn = KVM_PFN_ERR_FAULT;
                write_unlock_irq(&gpc->lock);

                spin_lock(&kvm->gpc_lock);
                list_del(&gpc->list);
                spin_unlock(&kvm->gpc_lock);

                gpc_unmap(old_pfn, old_khva);
        }
}