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: Thu, 20 Jun 2024 16:42:01 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: Sean Christopherson <seanjc@...gle.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Josh Triplett <josh@...htriplett.org>, 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 Fri, Jun 21, 2024 at 12:38:21AM +0200, Paolo Bonzini wrote:
> 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().

We only need smp_mb(), which is supplied by the srcu_read_lock()
function.  For now, anyway.  If we ever figure out how to get by with
lighter-weight ordering for srcu_read_lock(), then we will add an smp_mb()
to smp_mb__after_srcu_read_lock() to compensate.

							Thanx, Paul

> 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();
>