linux-kernel - Re: [PATCH v13 16/40] KVM: arm64: Manage GCS access and registers for guests

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Message-ID: <37fbc082-6bda-46e3-9ee7-9240b41f26fd@sirena.org.uk>
Date: Wed, 2 Oct 2024 19:24:12 +0100
From: Mark Brown <broonie@...nel.org>
To: Marc Zyngier <maz@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Arnd Bergmann <arnd@...db.de>, Oleg Nesterov <oleg@...hat.com>,
	Eric Biederman <ebiederm@...ssion.com>,
	Shuah Khan <shuah@...nel.org>,
	"Rick P. Edgecombe" <rick.p.edgecombe@...el.com>,
	Deepak Gupta <debug@...osinc.com>, Ard Biesheuvel <ardb@...nel.org>,
	Szabolcs Nagy <Szabolcs.Nagy@....com>, Kees Cook <kees@...nel.org>,
	"H.J. Lu" <hjl.tools@...il.com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Florian Weimer <fweimer@...hat.com>,
	Christian Brauner <brauner@...nel.org>,
	Thiago Jung Bauermann <thiago.bauermann@...aro.org>,
	Ross Burton <ross.burton@....com>,
	David Spickett <david.spickett@....com>,
	Yury Khrustalev <yury.khrustalev@....com>,
	Wilco Dijkstra <wilco.dijkstra@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
	kvmarm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-mm@...ck.org,
	linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v13 16/40] KVM: arm64: Manage GCS access and registers
 for guests

On Wed, Oct 02, 2024 at 04:55:25PM +0100, Marc Zyngier wrote:
> Marc Zyngier <maz@...nel.org> wrote:

> > > +	if (!kvm_has_gcs(kvm))
> > > +		kvm->arch.fgu[HFGxTR_GROUP] |= (HFGxTR_EL2_nGCS_EL0 |
> > > +						HFGxTR_EL2_nGCS_EL1);

> > Why are you still allowing the GCS instructions when GCS isn't
> > enabled?

> Scratch that, they are NOPs when GCS isn't enabled, so there shouldn't
> be any need for extra traps.

They are, though really they should UNDEF if GCS isn't there (which I
had thought was what you were referencing here).  Equally we only have
traps for a subset of GCS instructions and it's not like there aren't a
whole bunch of untrappable extensions anyway so it's not clear it's
worth the effort just for that.

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