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

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Message-ID: <699c1105-df8e-4a08-ae6c-8c01a13e4a3d@sirena.org.uk>
Date:   Tue, 22 Aug 2023 18:01:25 +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 Tue, Aug 22, 2023 at 05:34:38PM +0100, Catalin Marinas wrote:
> On Wed, Aug 16, 2023 at 07:15:53PM +0100, Mark Brown wrote:

> > 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.

> Well, I'd say either we are clear about why it's (not) needed or we ask
> the architects to clarify the spec. I haven't checked your latest
> series but in principle I don't like adding barriers just because we are
> not sure they are needed (and I don't think having hardware eventually
> changes this).

I should probably also mention that another part of my thinking was that
when we implement GCS for EL1 we'll need to ensure that everything is
synced during the pivot of the EL1 GCS (each EL needs an independent
GCS).  We won't be able to rely on having an ERET there so it's going to
have more stringent requirements, I was partly punting to think things
through fully there.

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