Merge branches 'acpi-resources', 'acpi-battery', 'acpi-doc' and 'acpi-pnp'

[linux-2.6-block.git] / arch / x86 / vdso / vclock_gettime.c

diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index 40d2473836c923acc5705018bf3aebf50cfb12b8..9793322751e02f63ddba0d1b8fef5f21b0a4d502 100644 (file)
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -82,15 +82,18 @@ static notrace cycle_t vread_pvclock(int *mode)
        cycle_t ret;
        u64 last;
        u32 version;
-       u32 migrate_count;
        u8 flags;
        unsigned cpu, cpu1;
 
 
        /*
-        * When looping to get a consistent (time-info, tsc) pair, we
-        * also need to deal with the possibility we can switch vcpus,
-        * so make sure we always re-fetch time-info for the current vcpu.
+        * Note: hypervisor must guarantee that:
+        * 1. cpu ID number maps 1:1 to per-CPU pvclock time info.
+        * 2. that per-CPU pvclock time info is updated if the
+        *    underlying CPU changes.
+        * 3. that version is increased whenever underlying CPU
+        *    changes.
+        *
         */
        do {
                cpu = __getcpu() & VGETCPU_CPU_MASK;
@@ -99,27 +102,20 @@ static notrace cycle_t vread_pvclock(int *mode)
                 * __getcpu() calls (Gleb).
                 */
 
-               /* Make sure migrate_count will change if we leave the VCPU. */
-               do {
-                       pvti = get_pvti(cpu);
-                       migrate_count = pvti->migrate_count;
-
-                       cpu1 = cpu;
-                       cpu = __getcpu() & VGETCPU_CPU_MASK;
-               } while (unlikely(cpu != cpu1));
+               pvti = get_pvti(cpu);
 
                version = __pvclock_read_cycles(&pvti->pvti, &ret, &flags);
 
                /*
                 * Test we're still on the cpu as well as the version.
-                * - We must read TSC of pvti's VCPU.
-                * - KVM doesn't follow the versioning protocol, so data could
-                *   change before version if we left the VCPU.
+                * We could have been migrated just after the first
+                * vgetcpu but before fetching the version, so we
+                * wouldn't notice a version change.
                 */
-               smp_rmb();
-       } while (unlikely((pvti->pvti.version & 1) ||
-                         pvti->pvti.version != version ||
-                         pvti->migrate_count != migrate_count));
+               cpu1 = __getcpu() & VGETCPU_CPU_MASK;
+       } while (unlikely(cpu != cpu1 ||
+                         (pvti->pvti.version & 1) ||
+                         pvti->pvti.version != version));
 
        if (unlikely(!(flags & PVCLOCK_TSC_STABLE_BIT)))
                *mode = VCLOCK_NONE;