[PATCH] KVM: X86: Avoid to handle first-time write when updating the pv stuffs each time

[Wanpeng Li <kernellwp@gmail.com>]

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

There is a logic to handle first-time write when updating the 
pvclock/wall clock/steal time shared memory pages each time, 
actually we should do this logic during pv stuffs setup if we 
suspect the version-field can't be guranteed to be initialized 
to an even number by the guest. This patch fixes it by handling 
the first-time write of pvclock/steal time during setup since 
the update is frequent, and keeping the wall clock since it is 
rare updating.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
 arch/x86/kvm/x86.c | 29 ++++++++++++++++++++++-------
 1 file changed, 22 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4552427..19311e0 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1833,9 +1833,6 @@ static void kvm_setup_pvclock_page(struct kvm_vcpu *v)
 	 */
 	BUILD_BUG_ON(offsetof(struct pvclock_vcpu_time_info, version) != 0);
 
-	if (guest_hv_clock.version & 1)
-		++guest_hv_clock.version;  /* first time write, random junk */
-
 	vcpu->hv_clock.version = guest_hv_clock.version + 1;
 	kvm_write_guest_cached(v->kvm, &vcpu->pv_time,
 				&vcpu->hv_clock,
@@ -2126,9 +2123,6 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
 
 	vcpu->arch.st.steal.preempted = 0;
 
-	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,
@@ -2256,8 +2250,19 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		     &vcpu->arch.pv_time, data & ~1ULL,
 		     sizeof(struct pvclock_vcpu_time_info)))
 			vcpu->arch.pv_time_enabled = false;
-		else
+		else {
+			struct pvclock_vcpu_time_info guest_hv_clock;
+
 			vcpu->arch.pv_time_enabled = true;
+			if (unlikely(kvm_read_guest_cached(vcpu->kvm, &vcpu->arch.pv_time,
+				&guest_hv_clock, sizeof(guest_hv_clock))))
+				break;
+			if (guest_hv_clock.version & 1)
+				++guest_hv_clock.version;  /* first time write, random junk */
+			kvm_write_guest_cached(vcpu->kvm, &vcpu->arch.pv_time,
+				&vcpu->arch.hv_clock,
+				sizeof(vcpu->arch.hv_clock.version));
+		}
 
 		break;
 	}
@@ -2283,6 +2288,16 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		if (!(data & KVM_MSR_ENABLED))
 			break;
 
+		if (unlikely(kvm_read_guest_cached(vcpu->kvm, &vcpu->arch.st.stime,
+			&vcpu->arch.st.steal, sizeof(struct kvm_steal_time))))
+			break;
+
+		if (vcpu->arch.st.steal.version & 1)
+			vcpu->arch.st.steal.version += 1;  /* first time write, random junk */
+
+		kvm_write_guest_cached(vcpu->kvm, &vcpu->arch.st.stime,
+			&vcpu->arch.st.steal, sizeof(struct kvm_steal_time));
+
 		kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
 
 		break;
-- 
2.7.4