linux-kernel - Re: [PATCH 7/8] arm64: cpufeature: Relax checks for AArch32 support at EL[0-2]

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Date:   Wed, 15 Apr 2020 13:29:26 +0100
From:   Will Deacon <will@...nel.org>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, mark.rutland@....com, maz@...nel.org,
        anshuman.khandual@....com, catalin.marinas@....com,
        saiprakash.ranjan@...eaurora.org, dianders@...omium.org,
        kernel-team@...roid.com
Subject: Re: [PATCH 7/8] arm64: cpufeature: Relax checks for AArch32 support
 at EL[0-2]

On Wed, Apr 15, 2020 at 12:37:31PM +0100, Suzuki K Poulose wrote:
> On 04/15/2020 11:58 AM, Will Deacon wrote:
> > On Wed, Apr 15, 2020 at 11:50:58AM +0100, Suzuki K Poulose wrote:
> > > On 04/14/2020 10:31 PM, Will Deacon wrote:
> > > > We don't need to be quite as strict about mismatched AArch32 support,
> > > > which is good because the friendly hardware folks have been busy
> > > > mismatching this to their hearts' content.
> > > > 
> > > >     * We don't care about EL2 or EL3 (there are silly comments concerning
> > > >       the latter, so remove those)
> > > > 
> > > >     * EL1 support is gated by the ARM64_HAS_32BIT_EL1 capability and handled
> > > >       gracefully when a mismatch occurs
> > > > 
> > > >     * EL1 support is gated by the ARM64_HAS_32BIT_EL0 capability and handled
> > > 
> > > s/EL1/EL0
> > > 
> > > >       gracefully when a mismatch occurs
> > > > 
> > > > Relax the AArch32 checks to FTR_NONSTRICT.
> > > 
> > > Agreed. We should do something similar for the features exposed by the
> > > ELF_HWCAP, of course in a separate series.
> > 
> > Hmm, I didn't think we needed to touch the HWCAPs, as they're derived from
> > the sanitised feature register values. What am I missing?
> 
> sorry, that was cryptic. I was suggesting to relax the ftr fields to
> NONSTRICT for the fields covered by ELF HWCAPs (and other CPU hwcaps).

Ah, gotcha. Given that the HWCAPs usually describe EL0 features, I say we
can punt this down the road until people give us hardware with mismatched
AArch32 at EL0.

Will