linux-kernel - Re: [PATCH v10 19/40] arm64/gcs: Context switch GCS state for EL0

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Message-ID: <3b316422-7f88-4f5d-a691-eb9209ec4ba9@sirena.org.uk>
Date: Tue, 20 Aug 2024 18:56:19 +0100
From: Mark Brown <broonie@...nel.org>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Will Deacon <will@...nel.org>, Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marc Zyngier <maz@...nel.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>,
	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 v10 19/40] arm64/gcs: Context switch GCS state for EL0

On Mon, Aug 19, 2024 at 04:44:52PM +0100, Mark Brown wrote:
> On Mon, Aug 19, 2024 at 12:46:13PM +0100, Catalin Marinas wrote:
> > On Thu, Aug 01, 2024 at 01:06:46PM +0100, Mark Brown wrote:

> > > +	/*
> > > +	 * Ensure that GCS changes are observable by/from other PEs in
> > > +	 * case of migration.
> > > +	 */
> > > +	if (task_gcs_el0_enabled(current) || task_gcs_el0_enabled(next))
> > > +		gcsb_dsync();

> > Could we do the sysreg writing under this 'if' block? If no app is using
> > GCS (which would be the case for a while), it looks like unnecessary
> > sysreg accesses.

> Yes, that should be fine I think.

I forgot when writing the above that we always allow reads from
GCSPR_EL0 in order to avoid corner cases for unwinders in the case of
asynchronous disable.  I'd expect that to be cheap to access though.

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