linux-kernel - Re: [PATCH v5 01/17] arm64: add pointer authentication register bits

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Message-ID: <20181012095008.swtk4gazlrc6rwdj@lakrids.cambridge.arm.com>
Date:   Fri, 12 Oct 2018 10:50:08 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     Kristina Martsenko <kristina.martsenko@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Adam Wallis <awallis@...eaurora.org>,
        Amit Kachhap <Amit.Kachhap@....com>,
        Andrew Jones <drjones@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoffer Dall <christoffer.dall@....com>,
        Dave P Martin <Dave.Martin@....com>,
        Jacob Bramley <jacob.bramley@....com>,
        Kees Cook <keescook@...omium.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        "Suzuki K . Poulose" <suzuki.poulose@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 01/17] arm64: add pointer authentication register bits

On Fri, Oct 12, 2018 at 09:56:05AM +0100, Will Deacon wrote:
> On Fri, Oct 12, 2018 at 09:53:54AM +0100, Mark Rutland wrote:
> > On Thu, Oct 11, 2018 at 05:28:14PM +0100, Will Deacon wrote:
> > > On Fri, Oct 05, 2018 at 09:47:38AM +0100, Kristina Martsenko wrote:
> > 
> > > > +#define ESR_ELx_EC_PAC		(0x09)
> > > 
> > > Really minor nit: but shouldn't this be ESR_EL2_EC_PAC, since this trap
> > > can't occur at EL1 afaict?
> > 
> > It can also be taken to EL3 dependent on SCR_EL3.API.
> > 
> > We use ESR_ELx_EC_<foo> for other exceptions that can't be taken to EL1
> > (e.g. ESR_ELx_EC_SMC{32,64}), so I think it would be more consistent to
> > leave this as ESR_ELx_EC_PAC rather than ESR_EL2_EC_PAC.
> 
> Fair enough, but if we grow a different EC for ESR_EL1 that uses encoding
> 0x09, this all falls apart.

We haven't had overlapping encodings so far, and if we did, we'd want to
apply some policy to all of these definitions, no?

> At the very list, maybe we should comment those that are EL2 or higher
> with /* EL2 and above */ or just fix the misnomer and drop the useless
> _ELx_ part of the names completely.

A comment sounds fine to me.

I'm not sure that s/_ELx// buys us any clarity, though; I don't think
that ESR_EC_PAC is clearly more constrained than ESR_ELx_EC_PAC.

Thanks,
Mark.