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

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Date: Fri, 21 Jun 2024 00:38:21 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: 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: Re: [PATCH 4/5] KVM: x86: Ensure a full memory barrier is emitted in
 the VM-Exit path

On 3/9/24 02:09, Sean Christopherson wrote:
> 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().

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();