[PATCH v2] use guest value of version field in kvmclock

[Glauber Costa]

Avi noticed that we have to use guest's value for the version field,
instead of keeping track of it ourselves. If we don't do that,
the following situation can arise:

 vcpu->arch.hv_clock.version is initialized to zero.
 Guest reads version (result: 2)
 Guest starts reading data
 Live migration; vcpu->arch.hv_clock.version is zeroed
 Steal time update; vcpu->arch.hv_clock.version += 2; write to guest
 Guest continues reading data
 Guest reads version (result: 2)

Signed-off-by: Glauber Costa <glommer@xxxxxxxxxx>
CC: Avi Kivity <avi@xxxxxxxxxx>

---
v2: don't get rid of the actual values =)
---
 arch/x86/kvm/x86.c |    9 ++++++---
 1 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c39ab4a..5707405 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1151,15 +1151,18 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 	vcpu->last_guest_tsc = tsc_timestamp;
 	vcpu->hv_clock.flags = 0;
 
+	shared_kaddr = kmap_atomic(vcpu->time_page, KM_USER0);
+
 	/*
 	 * 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.
+	 * state, we just increase by 2 at the end. We use the guest's value as
+	 * a basis to make it migration-safe.
 	 */
+	memcpy(&vcpu->hv_clock.version, shared_kaddr + vcpu->time_offset,
+	       sizeof(vcpu->hv_clock.version));
 	vcpu->hv_clock.version += 2;
 
-	shared_kaddr = kmap_atomic(vcpu->time_page, KM_USER0);
-
 	memcpy(shared_kaddr + vcpu->time_offset, &vcpu->hv_clock,
 	       sizeof(vcpu->hv_clock));
 
-- 
1.7.2.3

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