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 <pbonzini@redhat.com>
diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h
index 7c1c895..d019f0c 100644
--- a/arch/x86/include/asm/pvclock.h
+++ b/arch/x86/include/asm/pvclock.h
@@ -25,6 +25,24 @@
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 __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 {