[PATCH] kvm: x86: Don't dirty guest memory on every vcpu_put()

[Jim Mattson <jmattson@google.com>]

Beginning with commit 0b9f6c4615c99 ("x86/kvm: Support the vCPU
preemption check"), the KVM_VCPU_PREEMPTED flag is set in the guest
copy of the kvm_steal_time struct on every call to vcpu_put(). As a
result, guest memory is dirtied on every call to vcpu_put(), even when
the VM is quiescent.

To avoid dirtying guest memory unnecessarily, don't bother setting the
flag in the guest copy of the struct if it is already set in the
kernel copy of the struct.

If a different vCPU thread clears the guest copy of the flag, it will
no longer get reset on the next call to vcpu_put, but it's not clear
that resetting the flag in this case was intentional to begin with.

Signed-off-by: Jim Mattson <jmattson@google.com>
Tested-by: Kevin Mcgaire <kevinmcgaire@google.com>
Reviewed-by: Ben Gardon <bgardon@google.com>
Reviewed-by: Oliver Upton <oupton@google.com>

---
 arch/x86/kvm/x86.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index cf917139de6b..3dc17b173f88 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3504,6 +3504,9 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu)
 	if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED))
 		return;
 
+	if (vcpu->arch.st.steal.preempted & KVM_VCPU_PREEMPTED)
+		return;
+
 	vcpu->arch.st.steal.preempted = KVM_VCPU_PREEMPTED;
 
 	kvm_write_guest_offset_cached(vcpu->kvm, &vcpu->arch.st.stime,
-- 
2.25.0.rc1.283.g88dfdc4193-goog