kvm: x86: Fix kvm clock versioning.

kvm updates the version number for the guest paravirt clock structure by
incrementing the version of its private copy. It does not read the guest
version, so will write version = 2 in the first update for every new VM,
including after restoring a saved state. If guest state is saved during
reading the clock, it could read and accept struct fields and guest TSC
from two different updates. This changes the code to increment the guest
version and write it back.

Signed-off-by: Owen Hofmann <osh@google.com>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e0260ccd78a4ab4775e8399aba5dd4374f1f09ba..8bf37d0ab40460f8457a04053e77a8af31fb3022 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1636,16 +1636,16 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
        vcpu->hv_clock.system_time = kernel_ns + v->kvm->arch.kvmclock_offset;
        vcpu->last_guest_tsc = tsc_timestamp;
 
+       if (unlikely(kvm_read_guest_cached(v->kvm, &vcpu->pv_time,
+               &guest_hv_clock, sizeof(guest_hv_clock))))
+               return 0;
+
        /*
         * The interface expects us to write an even number signaling that the
         * update is finished. Since the guest won't see the intermediate
         * state, we just increase by 2 at the end.
         */
-       vcpu->hv_clock.version += 2;
-
-       if (unlikely(kvm_read_guest_cached(v->kvm, &vcpu->pv_time,
-               &guest_hv_clock, sizeof(guest_hv_clock))))
-               return 0;
+       vcpu->hv_clock.version = guest_hv_clock.version + 2;
 
        /* retain PVCLOCK_GUEST_STOPPED if set in guest copy */
        pvclock_flags = (guest_hv_clock.flags & PVCLOCK_GUEST_STOPPED);