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Message-ID: <bbd59a2c0897d8ca642ea8c4787b829190e75a4d.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>, Paul Durrant <paul@xen.org>
Cc: 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 20:27:37 -0800
In-Reply-To: <ZcMFb1epchA7Mbzo@google.com>
References: <20240115125707.1183-1-paul@xen.org>
	 <20240115125707.1183-19-paul@xen.org> <ZcMFb1epchA7Mbzo@google.com>
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On Tue, 2024-02-06 at 20:22 -0800, Sean Christopherson wrote:
> On Mon, Jan 15, 2024, Paul Durrant wrote:
> > From: Paul Durrant <pdurrant@amazon.com>
> >=20
> > Taking a write lock on a pfncache will be disruptive if the cache is
>=20
> *Unnecessarily* taking a write lock.

No. Taking a write lock will be disrupting.

Unnecessarily taking a write lock will be unnecessarily disrupting.

Taking a write lock on a Thursday will be disrupting on a Thursday.

But the key is that if the cache is heavily used, the user gets
disrupted.


> =C2=A0 Please save readers a bit of brain power
> and explain that this is beneificial when there are _unrelated_ invalidat=
ion.

I don't understand what you're saying there. Paul's sentence did have
an implicit "...so do that less then", but that didn't take much brain
power to infer.

> > heavily used (which only requires a read lock). Hence, in the MMU notif=
ier
> > 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).
>=20
> This doesn't have any dependency on this series, does it?=C2=A0 I.e. this=
 should be
> posted separately, and preferably with some performance data.=C2=A0 Not h=
aving data
> isn't a sticking point, but it would be nice to verify that this isn't a
> pointless optimization.

No fundamental dependency, no. But it was triggered by the previous
patch, which makes kvm_xen_set_evtchn_fast() use read_trylock() and
makes it take the slow path when there's contention. It lives here just
fine as part of the series.


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