[PATCH] kvm: robustify steal time record

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpeng.li@hotmail.com>

Guest should only trust data to be valid when version haven't changed 
before and after reads of steal time. Besides not changing, it has to 
be an even number. Hypervisor may write an odd number to version field 
to indicate that an update is in progress.

kvm_steal_clock() in guest has already done the read side, make write 
side in hypervisor more robust by following the above rule.

Reviewed-by: Wincy Van <fanwenyi0529@gmail.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
 arch/x86/kvm/x86.c |   18 +++++++++++++++++-
 1 files changed, 17 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 96e81d2..f935868 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2010,11 +2010,27 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
 	if (unlikely(kvm_read_guest_cached(vcpu->kvm, &vcpu->arch.st.stime,
 		&vcpu->arch.st.steal, sizeof(struct kvm_steal_time))))
 		return;
+
+	if (vcpu->arch.st.steal.version & 1)
+		vcpu->arch.st.steal.version += 1;  /* first time write, random junk */
+
+	vcpu->arch.st.steal.version += 1;
+
+	kvm_write_guest_cached(vcpu->kvm, &vcpu->arch.st.stime,
+		&vcpu->arch.st.steal, sizeof(struct kvm_steal_time));
+
+	smp_wmb();
 
 	vcpu->arch.st.steal.steal += current->sched_info.run_delay -
 		vcpu->arch.st.last_steal;
 	vcpu->arch.st.last_steal = current->sched_info.run_delay;
-	vcpu->arch.st.steal.version += 2;
+
+	kvm_write_guest_cached(vcpu->kvm, &vcpu->arch.st.stime,
+		&vcpu->arch.st.steal, sizeof(struct kvm_steal_time));
+
+	smp_wmb();
+
+	vcpu->arch.st.steal.version += 1;
 
 	kvm_write_guest_cached(vcpu->kvm, &vcpu->arch.st.stime,
 		&vcpu->arch.st.steal, sizeof(struct kvm_steal_time));
-- 
1.7.1