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pvclock: introduce seqcount-like API

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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: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2016-06-09 13:06:08 +02:00
parent 96b585267f
commit 3aed64f6d3
3 changed files with 31 additions and 34 deletions
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9
arch/x86/entry/vdso/vclock_gettime.c
View File

@ -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 +

39
arch/x86/include/asm/pvclock.h
View File

@ -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,
src->tsc_shift);
*cycles = src->system_time + offset;
*flags = src->flags;
return version;
u64 delta = rdtsc_ordered() - src->tsc_timestamp;
cycle_t offset = pvclock_scale_delta(delta, src->tsc_to_system_mul,
src->tsc_shift);
return src->system_time + offset;
}
struct pvclock_vsyscall_time_info {

17
arch/x86/kernel/pvclock.c
View File

@ -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;