linux-kernel - Re: [PATCH v12 18/20] KVM: pfncache: check the need for invalidation under read lock first

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Message-ID: <ZcMFb1epchA7Mbzo@google.com>
Date: Tue, 6 Feb 2024 20:22:07 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paul Durrant <paul@....org>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, David Woodhouse <dwmw2@...radead.org>, Shuah Khan <shuah@...nel.org>, 
	kvm@...r.kernel.org, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v12 18/20] KVM: pfncache: check the need for invalidation
 under read lock first

On Mon, Jan 15, 2024, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@...zon.com>
> 
> Taking a write lock on a pfncache will be disruptive if the cache is

*Unnecessarily* taking a write lock.  Please save readers a bit of brain power
and explain that this is beneificial when there are _unrelated_ invalidation.

> heavily used (which only requires a read lock). Hence, in the MMU notifier
> callback, take read locks on caches to check for a match; only taking a
> write lock to actually perform an invalidation (after a another check).

This doesn't have any dependency on this series, does it?  I.e. this should be
posted separately, and preferably with some performance data.  Not having data
isn't a sticking point, but it would be nice to verify that this isn't a
pointless optimization.