linux-kernel - Re: [PATCH V2 5/7] arm64/cpufeature: Add field details for ID_AA64DFR1

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Message-ID: <20241210170519.GA16075@willie-the-truck>
Date: Tue, 10 Dec 2024 17:05:20 +0000
From: Will Deacon <will@...nel.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Anshuman Khandual <anshuman.khandual@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Mark Brown <broonie@...nel.org>, kvmarm@...ts.linux.dev
Subject: Re: [PATCH V2 5/7] arm64/cpufeature: Add field details for
 ID_AA64DFR1_EL1 register

On Tue, Dec 10, 2024 at 04:56:25PM +0000, Mark Rutland wrote:
> On Tue, Dec 10, 2024 at 04:41:44PM +0000, Will Deacon wrote:
> > On Mon, Oct 28, 2024 at 11:04:24AM +0530, Anshuman Khandual wrote:
> > > +static const struct arm64_ftr_bits ftr_id_aa64dfr1[] = {
> > > +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_ABL_CMPs_SHIFT, 8, 0),
> > > +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_DPFZS_SHIFT, 4, 0),
> > > +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_EBEP_SHIFT, 4, 0),
> > > +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_ITE_SHIFT, 4, 0),
> > > +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_ABLE_SHIFT, 4, 0),
> > > +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_PMICNTR_SHIFT, 4, 0),
> > > +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_SPMU_SHIFT, 4, 0),
> > > +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_CTX_CMPs_SHIFT, 8, 0),
> > > +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_WRPs_SHIFT, 8, 0),
> > > +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_BRPs_SHIFT, 8, 0),
> > > +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_SYSPMUID_SHIFT, 8, 0),
> > > +	ARM64_FTR_END,
> > > +};
> > 
> > I think I mentioned this on an earlier series, but it would be useful to
> > see some justification in the commit message as to why some of these
> > features are considered STRICT vs NONSTRICT and why LOWER_SAFE is
> > preferred over EXACT.
> > 
> > For example, why is EBEP strict whereas other PMU-related fields aren't?
> > Why is the CTX_CMPs field treated differently to the same field in DFR0?
> > 
> > I'm not saying the above table is wrong, it just looks arbitrary without
> > the justification.
> 
> FWIW, Anshuman and I discussed that on the v1 thread, after this v2
> thread was posted. Anshuman promised to provide some rationale and make
> some updates in the next version (i.e. v3):
> 
>   https://lore.kernel.org/linux-arm-kernel/8efe902c-8b9f-494a-b9da-430d8ced32ef@arm.com/

Perfect! I'll wait for the v3, then.

Will