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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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39
arch/x86/include/asm/pvclock.h
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@@ -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 {