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

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On 3/9/24 02:09, Sean Christopherson wrote:
From: Yan Zhao <yan.y.zhao@xxxxxxxxx>

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().

Do you really need mfence or is a locked operation enough? mfence is mb(), not smp_mb().

Paolo

+	/*
+	 * 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();





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