powerpc/paravirt: vcpu_is_preempted() commentary

[thirdparty/linux.git] / arch / powerpc / include / asm / paravirt.h

/* SPDX-License-Identifier: GPL-2.0-or-later */
#ifndef _ASM_POWERPC_PARAVIRT_H
#define _ASM_POWERPC_PARAVIRT_H

#include <linux/jump_label.h>
#include <asm/smp.h>
#ifdef CONFIG_PPC64
#include <asm/paca.h>
#include <asm/hvcall.h>
#endif

#ifdef CONFIG_PPC_SPLPAR
#include <linux/smp.h>
#include <asm/kvm_guest.h>
#include <asm/cputhreads.h>

DECLARE_STATIC_KEY_FALSE(shared_processor);

static inline bool is_shared_processor(void)
{
	return static_branch_unlikely(&shared_processor);
}

/* If bit 0 is set, the cpu has been ceded, conferred, or preempted */
static inline u32 yield_count_of(int cpu)
{
	__be32 yield_count = READ_ONCE(lppaca_of(cpu).yield_count);
	return be32_to_cpu(yield_count);
}

/*
 * Spinlock code confers and prods, so don't trace the hcalls because the
 * tracing code takes spinlocks which can cause recursion deadlocks.
 *
 * These calls are made while the lock is not held: the lock slowpath yields if
 * it can not acquire the lock, and unlock slow path might prod if a waiter has
 * yielded). So this may not be a problem for simple spin locks because the
 * tracing does not technically recurse on the lock, but we avoid it anyway.
 *
 * However the queued spin lock contended path is more strictly ordered: the
 * H_CONFER hcall is made after the task has queued itself on the lock, so then
 * recursing on that lock will cause the task to then queue up again behind the
 * first instance (or worse: queued spinlocks use tricks that assume a context
 * never waits on more than one spinlock, so such recursion may cause random
 * corruption in the lock code).
 */
static inline void yield_to_preempted(int cpu, u32 yield_count)
{
	plpar_hcall_norets_notrace(H_CONFER, get_hard_smp_processor_id(cpu), yield_count);
}

static inline void prod_cpu(int cpu)
{
	plpar_hcall_norets_notrace(H_PROD, get_hard_smp_processor_id(cpu));
}

static inline void yield_to_any(void)
{
	plpar_hcall_norets_notrace(H_CONFER, -1, 0);
}
#else
static inline bool is_shared_processor(void)
{
	return false;
}

static inline u32 yield_count_of(int cpu)
{
	return 0;
}

extern void ___bad_yield_to_preempted(void);
static inline void yield_to_preempted(int cpu, u32 yield_count)
{
	___bad_yield_to_preempted(); /* This would be a bug */
}

extern void ___bad_yield_to_any(void);
static inline void yield_to_any(void)
{
	___bad_yield_to_any(); /* This would be a bug */
}

extern void ___bad_prod_cpu(void);
static inline void prod_cpu(int cpu)
{
	___bad_prod_cpu(); /* This would be a bug */
}

#endif

#define vcpu_is_preempted vcpu_is_preempted
static inline bool vcpu_is_preempted(int cpu)
{
	/*
	 * The dispatch/yield bit alone is an imperfect indicator of
	 * whether the hypervisor has dispatched @cpu to run on a physical
	 * processor. When it is clear, @cpu is definitely not preempted.
	 * But when it is set, it means only that it *might* be, subject to
	 * other conditions. So we check other properties of the VM and
	 * @cpu first, resorting to the yield count last.
	 */

	/*
	 * Hypervisor preemption isn't possible in dedicated processor
	 * mode by definition.
	 */
	if (!is_shared_processor())
		return false;

#ifdef CONFIG_PPC_SPLPAR
	if (!is_kvm_guest()) {
		int first_cpu = cpu_first_thread_sibling(smp_processor_id());

		/*
		 * The PowerVM hypervisor dispatches VMs on a whole core
		 * basis. So we know that a thread sibling of the local CPU
		 * cannot have been preempted by the hypervisor, even if it
		 * has called H_CONFER, which will set the yield bit.
		 */
		if (cpu_first_thread_sibling(cpu) == first_cpu)
			return false;
	}
#endif

	if (yield_count_of(cpu) & 1)
		return true;
	return false;
}

static inline bool pv_is_native_spin_unlock(void)
{
	return !is_shared_processor();
}

#endif /* _ASM_POWERPC_PARAVIRT_H */