x86: kvm: Revert "remove sched notifier for cross-cpu migrations"

The following point:

    2. per-CPU pvclock time info is updated if the
       underlying CPU changes.

Is not true anymore since "KVM: x86: update pvclock area conditionally,
on cpu migration".

Add task migration notification back.

Problem noticed by Andy Lutomirski.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
CC: stable@kernel.org # 3.11+

1 files changed, 8 insertions, 8 deletions

diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index 9793322751e0..30933760ee5f 100644
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -82,18 +82,15 @@ static notrace cycle_t vread_pvclock(int *mode)
         cycle_t ret;
         u64 last;
         u32 version;
+        u32 migrate_count;
         u8 flags;
         unsigned cpu, cpu1;
 
 
         /*
-         * 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.
-         *
+         * 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.
          */
         do {
                 cpu = __getcpu() & VGETCPU_CPU_MASK;
@@ -104,6 +101,8 @@ static notrace cycle_t vread_pvclock(int *mode)
 
                 pvti = get_pvti(cpu);
 
+                migrate_count = pvti->migrate_count;
+
                 version = __pvclock_read_cycles(&pvti->pvti, &ret, &flags);
 
                 /*
@@ -115,7 +114,8 @@ static notrace cycle_t vread_pvclock(int *mode)
                 cpu1 = __getcpu() & VGETCPU_CPU_MASK;
         } while (unlikely(cpu != cpu1 ||
                           (pvti->pvti.version & 1) ||
-                          pvti->pvti.version != version));
+                          pvti->pvti.version != version ||
+                          pvti->migrate_count != migrate_count));
 
         if (unlikely(!(flags & PVCLOCK_TSC_STABLE_BIT)))
                 *mode = VCLOCK_NONE;