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Commit 3aed64f6 authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

pvclock: introduce seqcount-like API 



The version field in struct pvclock_vcpu_time_info basically implements
a seqcount.  Wrap it with the usual read_begin and read_retry functions,
and use these APIs instead of peppering the code with smp_rmb()s.
While at it, change it to the more pedantically correct virt_rmb().

With this change, __pvclock_read_cycles can be simplified noticeably.

Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 96b58526

arch/x86/entry/vdso/vclock_gettime.c

+2 −7
Original line number Diff line number Diff line
@@ -123,9 +123,7 @@ static notrace cycle_t vread_pvclock(int *mode) 
	 */



	do {

		version = pvti->version;



		smp_rmb();

		version = pvclock_read_begin(pvti);



		if (unlikely(!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))) {

			*mode = VCLOCK_NONE;
@@ -137,10 +135,7 @@ static notrace cycle_t vread_pvclock(int *mode) 
		pvti_tsc_shift = pvti->tsc_shift;

		pvti_system_time = pvti->system_time;

		pvti_tsc = pvti->tsc_timestamp;



		/* Make sure that the version double-check is last. */

		smp_rmb();

	} while (unlikely((version & 1) || version != pvti->version));

	} while (pvclock_read_retry(pvti, version));



	delta = tsc - pvti_tsc;

	ret = pvti_system_time +

arch/x86/include/asm/pvclock.h

+23 −16
Original line number Diff line number Diff line
@@ -25,6 +25,24 @@ void pvclock_resume(void); 


void pvclock_touch_watchdogs(void);



static __always_inline

unsigned pvclock_read_begin(const struct pvclock_vcpu_time_info *src)

{

	unsigned version = src->version & ~1;

	/* Make sure that the version is read before the data. */

	virt_rmb();

	return version;

}



static __always_inline

bool pvclock_read_retry(const struct pvclock_vcpu_time_info *src,

			unsigned version)

{

	/* Make sure that the version is re-read after the data. */

	virt_rmb();

	return unlikely(version != src->version);

}



/*

 * Scale a 64-bit delta by scaling and multiplying by a 32-bit fraction,

 * yielding a 64-bit result.
@@ -69,23 +87,12 @@ static inline u64 pvclock_scale_delta(u64 delta, u32 mul_frac, int shift) 
}



static __always_inline

unsigned __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src,

			       cycle_t *cycles, u8 *flags)

cycle_t __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src)

{

	unsigned version;

	cycle_t offset;

	u64 delta;



	version = src->version;

	/* Make the latest version visible */

	smp_rmb();



	delta = rdtsc_ordered() - src->tsc_timestamp;

	offset = pvclock_scale_delta(delta, src->tsc_to_system_mul,

	u64 delta = rdtsc_ordered() - src->tsc_timestamp;

	cycle_t offset = pvclock_scale_delta(delta, src->tsc_to_system_mul,

					     src->tsc_shift);

	*cycles = src->system_time + offset;

	*flags = src->flags;

	return version;

	return src->system_time + offset;

}



struct pvclock_vsyscall_time_info {

arch/x86/kernel/pvclock.c

+6 −11
Original line number Diff line number Diff line
@@ -64,14 +64,9 @@ u8 pvclock_read_flags(struct pvclock_vcpu_time_info *src) 
	u8 flags;



	do {

		version = src->version;

		/* Make the latest version visible */

		smp_rmb();



		version = pvclock_read_begin(src);

		flags = src->flags;

		/* Make sure that the version double-check is last. */

		smp_rmb();

	} while ((src->version & 1) || version != src->version);

	} while (pvclock_read_retry(src, version));



	return flags & valid_flags;

}
@@ -84,10 +79,10 @@ cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src) 
	u8 flags;



	do {

		version = __pvclock_read_cycles(src, &ret, &flags);

		/* Make sure that the version double-check is last. */

		smp_rmb();

	} while ((src->version & 1) || version != src->version);

		version = pvclock_read_begin(src);

		ret = __pvclock_read_cycles(src);

		flags = src->flags;

	} while (pvclock_read_retry(src, version));



	if (unlikely((flags & PVCLOCK_GUEST_STOPPED) != 0)) {

		src->flags &= ~PVCLOCK_GUEST_STOPPED;