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Message-ID: <a817d64f3fe7b935a02e78df02dc0c6281e61af3.camel@infradead.org>
Subject: Re: [PATCH v12 18/20] KVM: pfncache: check the need for
 invalidation under read lock first
From: David Woodhouse <dwmw2@infradead.org>
To: Sean Christopherson <seanjc@google.com>
Cc: Paul Durrant <paul@xen.org>, Paolo Bonzini <pbonzini@redhat.com>, 
 Jonathan Corbet <corbet@lwn.net>, Thomas Gleixner <tglx@linutronix.de>,
 Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, Dave Hansen
 <dave.hansen@linux.intel.com>, x86@kernel.org, "H. Peter Anvin"
 <hpa@zytor.com>,  Shuah Khan <shuah@kernel.org>, kvm@vger.kernel.org,
 linux-doc@vger.kernel.org,  linux-kernel@vger.kernel.org,
 linux-kselftest@vger.kernel.org
Date: Tue, 06 Feb 2024 20:59:26 -0800
In-Reply-To: <ZcMLX5Omum3riZe8@google.com>
References: <20240115125707.1183-1-paul@xen.org>
	 <20240115125707.1183-19-paul@xen.org> <ZcMFb1epchA7Mbzo@google.com>
	 <bbd59a2c0897d8ca642ea8c4787b829190e75a4d.camel@infradead.org>
	 <ZcMLX5Omum3riZe8@google.com>
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On Tue, 2024-02-06 at 20:47 -0800, Sean Christopherson wrote:
>=20
> I'm saying this:
>=20
> =C2=A0 When processing mmu_notifier invalidations for gpc caches, pre-che=
ck for
> =C2=A0 overlap with the invalidation event while holding gpc->lock for re=
ad, and
> =C2=A0 only take gpc->lock for write if the cache needs to be invalidated=
=C2=A0 Doing
> =C2=A0 a pre-check without taking gpc->lock for write avoids unnecessaril=
y
> =C2=A0 contending the lock for unrelated invalidations, which is very ben=
eficial
> =C2=A0 for caches that are heavily used (but rarely subjected to mmu_noti=
fier
> =C2=A0 invalidations).
>=20
> is much friendlier to readers than this:
>=20
> =C2=A0 Taking a write lock on a pfncache will be disruptive if the cache =
is
> =C2=A0 heavily used (which only requires a read lock). Hence, in the MMU =
notifier
> =C2=A0 callback, take read locks on caches to check for a match; only tak=
ing a
> =C2=A0 write lock to actually perform an invalidation (after a another ch=
eck).

That's a somewhat subjective observation. I actually find the latter to
be far more succinct and obvious.

Actually... maybe I find yours harder because it isn't actually stating
the situation as I understand it. You said "unrelated invalidation" in
your first email, and "overlap with the invalidation event" in this
one... neither of which makes sense to me because there is no *other*
invalidation here.

We're only talking about the MMU notifier gratuitously taking the write
lock on a GPC that it *isn't* going to invalidate (the common case),
and that disrupting users which are trying to take the read lock on
that GPC.

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