[PATCH 4/5] KVM: x86: Ensure a full memory barrier is emitted in the VM-Exit path

[Sean Christopherson <seanjc@google.com>]

From: Yan Zhao <yan.y.zhao@intel.com>

Ensure a full memory barrier is emitted in the VM-Exit path, as a full
barrier is required on Intel CPUs to evict WC buffers.  This will allow
unconditionally honoring guest PAT on Intel CPUs that support self-snoop.

As srcu_read_lock() is always called in the VM-Exit path and it internally
has a smp_mb(), call smp_mb__after_srcu_read_lock() to avoid adding a
second fence and make sure smp_mb() is called without dependency on
implementation details of srcu_read_lock().

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
[sean: massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 276ae56dd888..69e815df1699 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11082,6 +11082,12 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 
 	kvm_vcpu_srcu_read_lock(vcpu);
 
+	/*
+	 * Call this to ensure WC buffers in guest are evicted after each VM
+	 * Exit, so that the evicted WC writes can be snooped across all cpus
+	 */
+	smp_mb__after_srcu_read_lock();
+
 	/*
 	 * Profile KVM exit RIPs:
 	 */
-- 
2.44.0.278.ge034bb2e1d-goog