linux-kernel - Re: [PATCH v4 18/36] arm64/gcs: Context switch GCS state for EL0

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Message-ID: <28a61b5f-db65-427e-8e92-60dd61549da5@sirena.org.uk>
Date:   Wed, 16 Aug 2023 19:15:53 +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>,
        Kees Cook <keescook@...omium.org>,
        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>,
        "H.J. Lu" <hjl.tools@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        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 v4 18/36] arm64/gcs: Context switch GCS state for EL0

On Fri, Aug 11, 2023 at 04:32:10PM +0100, Catalin Marinas wrote:
> On Mon, Aug 07, 2023 at 11:00:23PM +0100, Mark Brown wrote:

> > +		gcs_free(current);
> > +		current->thread.gcs_el0_mode = 0;
> > +		write_sysreg_s(0, SYS_GCSCRE0_EL1);
> > +		write_sysreg_s(0, SYS_GCSPR_EL0);
> > +	}
> > +}

> Do we need and isb() or there's one on this path? If it's only EL0
> making use of this register, we should be fine with the ERET before
> returning to user. Not sure whether the kernel uses this, GCSSTTR
> doesn't need it.

They're only used by EL0, at EL1 we do read GCSPR for signal handling
but AIUI that shouldn't be any more of an issue than it is for the
TPIDRs which we don't have a barrier for.  It's possible I'm
misunderstanding though.

> > +	/*
> > +	 * 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();

> What's this barrier for? The spec (at least the version I have) only
> talks about accesses, nothing to do with the registers that we context
> switch here.

Right, it's for the GCS memory rather than the registers.  I'm fairly
sure it's excessive but but was erring on the side of caution until I
have convinced myself that the interactions between GCS barriers and
regular barriers were doing the right thing, until we have physical
implementations to contend with I'd guess the practical impact will be
minimal.

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