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

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Date: Fri,  8 Mar 2024 17:09:28 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>, 
	Lai Jiangshan <jiangshanlai@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>, 
	Josh Triplett <josh@...htriplett.org>
Cc: kvm@...r.kernel.org, rcu@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Kevin Tian <kevin.tian@...el.com>, Yan Zhao <yan.y.zhao@...el.com>, 
	Yiwei Zhang <zzyiwei@...gle.com>
Subject: [PATCH 4/5] KVM: x86: Ensure a full memory barrier is emitted in the
 VM-Exit path

From: Yan Zhao <yan.y.zhao@...el.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@...hat.com>
Cc: Sean Christopherson <seanjc@...gle.com>
Cc: Kevin Tian <kevin.tian@...el.com>
Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
[sean: massage changelog]
Signed-off-by: Sean Christopherson <seanjc@...gle.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