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Date: Tue, 13 Oct 2015 10:40:26 +0100 From: Catalin Marinas <catalin.marinas@....com> To: Mark Rutland <mark.rutland@....com> Cc: "Suzuki K. Poulose" <Suzuki.Poulose@....com>, Vladimir.Murzin@....com, steve.capper@...aro.org, ard.biesheuvel@...aro.org, marc.zyngier@....com, will.deacon@....com, linux-kernel@...r.kernel.org, edward.nevill@...aro.org, aph@...hat.com, james.morse@....com, andre.przywara@....com, dave.martin@....com, linux-arm-kernel@...ts.infradead.org Subject: Re: [PATCH v2 07/22] arm64: Keep track of CPU feature registers On Mon, Oct 12, 2015 at 06:21:04PM +0100, Mark Rutland wrote: > On Mon, Oct 12, 2015 at 06:01:25PM +0100, Suzuki K. Poulose wrote: > > On 08/10/15 16:03, Catalin Marinas wrote: > > >On Thu, Oct 08, 2015 at 10:55:11AM +0100, Suzuki K. Poulose wrote: > > > > ... > > > > > >So we have three types of fields in these registers: > > > > > >a) features defined but not something we care about in Linux > > >b) reserved fields > > >c) features important to Linux > > > > > >I guess for (a), Linux may not even care if they don't match (though we > > >need to be careful which fields we ignore). As for (b), even if they > > >differ, since we don't know the meaning at this point, I think we should > > >just ignore them. If, for example, they add a feature that Linux doesn't > > >care about, they practically fall under the (a) category. > > > > > >Regarding exposing reserved CPUID fields to user, I assume we would > > >always return 0. > > > > Mark, > > > > Do you have any comments on this ? The list I have here is what you came > > up with in SANITY checks. > > My feeling was that we should play it safe with fields which are > currently reserved (warning if they differ for now). > > If they turn out to be irrelevant, it's simple to backport a patch to > ignore them, whereas if they matter we get instant visibility, which is > the entire point of the sanity checks. > > So I think we should warn if reserved fields differ. I'd rather have a > few spurious warnings until kernels get updated than miss an issue that > could have been dealt with and avoided. The warnings are indeed harmless. I think the main danger is when a field goes negative which means an existing feature without CPUID field allocated is removed (though I don't expect this for ARMv8). But you are right, let the warnings in for now, let's re-assess when/if a difference happens. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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