linux-kernel - Re: [PATCH] KVM: arm64: Only save S1PIE registers when dirty

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Message-ID: <50c5cdd2-fceb-44c4-aff1-dc98180161a1@sirena.org.uk>
Date: Mon, 4 Mar 2024 14:11:19 +0000
From: Mark Brown <broonie@...nel.org>
To: Marc Zyngier <maz@...nel.org>
Cc: Oliver Upton <oliver.upton@...ux.dev>,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Joey Gouly <joey.gouly@....com>,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: arm64: Only save S1PIE registers when dirty

On Sat, Mar 02, 2024 at 10:28:18AM +0000, Marc Zyngier wrote:
> Mark Brown <broonie@...nel.org> wrote:
> > On Fri, Mar 01, 2024 at 07:32:28PM +0000, Oliver Upton wrote:

> > > The overheads of guest exits are extremely configuration dependent, and
> > > on VHE the save/restore of EL1 state happens at vcpu_load() / vcpu_put()
> > > rather than every exit. There isn't a whole lot KVM can do to lessen the
> > > blow of sharing EL1 in the nVHE configuration.

> > > Looking a bit further out, the cost of traps will be dramatically higher
> > > when running as a guest hypervisor, so we'd want to avoid them if
> > > possible...

> > Indeed, but OTOH I got some complaints about adding more system register

> Complains from whom? I can't see anything in my inbox, so it my
> conclusion that these "issues" are not serious enough to be publicly
> mentioned.

This was you saying that adding more registers to be context switched
here needed special explanation, rather than just being the default and
generally unremarkable place to put context switching of registers for
EL0/1.

> If anything, I'm actually minded to remove existing instances of this
> stupid trapping, such as PAuth, which is entirely pointless.

That one was part of why it appeared that this sort of thing was what
you were asking for.  Especially given that there's nothing I can see
explaining why this would be deferred it's really unclear, I'd expect it
to be likely that those registers will be quite frequently accessed if
pointer authentication is in use.  Either it needs more explanation of
why it's special or it does seem like it should be removed.

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