From 3aed64f6d341cdb62bb2d6232589fb13448ce063 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Thu, 9 Jun 2016 13:06:08 +0200 Subject: [PATCH] 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: Paolo Bonzini --- arch/x86/entry/vdso/vclock_gettime.c | 9 ++----- arch/x86/include/asm/pvclock.h | 39 ++++++++++++++++------------ arch/x86/kernel/pvclock.c | 17 +++++------- 3 files changed, 31 insertions(+), 34 deletions(-) diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c index 2f02d23a05ef..db1e3b4c3693 100644 --- a/arch/x86/entry/vdso/vclock_gettime.c +++ b/arch/x86/entry/vdso/vclock_gettime.c @@ -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 + diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h index 7c1c89598688..d019f0cc80ec 100644 --- a/arch/x86/include/asm/pvclock.h +++ b/arch/x86/include/asm/pvclock.h @@ -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 { diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c index 06c58ce46762..3599404e3089 100644 --- a/arch/x86/kernel/pvclock.c +++ b/arch/x86/kernel/pvclock.c @@ -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; -- 2.20.1