linux-kernel - Re: [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD capability

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Date:   Tue, 15 Oct 2019 15:05:42 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Mark Rutland <Mark.Rutland@....com>,
        Suzuki Poulose <Suzuki.Poulose@....com>,
        Catalin Marinas <Catalin.Marinas@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "will@...nel.org" <will@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD
 capability

On Tue, Oct 15, 2019 at 12:30:15PM +0200, Ard Biesheuvel wrote:
> On Tue, 15 Oct 2019 at 12:25, Dave Martin <Dave.Martin@....com> wrote:
> >
> > On Mon, Oct 14, 2019 at 06:57:30PM +0200, Ard Biesheuvel wrote:

[...]

> > > All in-kernel NEON code checks whether the NEON is usable, so I'd
> > > expect that check to return 'false' if it is too early in the boot for
> > > the NEON to be used at all.
> >
> > My concern is that the check may be done once, at probe time, for crypto
> > drivers.  If probing happens before system_supports_fpsimd() has
> > stabilised, we may be stuck with the wrong probe decision.
> >
> > So: are crypto drivers and kernel_mode_neon() users definitely only
> > probed _after_ all early CPUs are up?
> >
> 
> Isn't SMP already up when initcalls are processed?

That was my original assumption when developing SVE.  I think I
convinced myself that it was valid, but it sounds worth reinvestigating.

Assuming the assumption _is_ valid, then dropping a suitable WARN() into
system_supports_fpsimd() or cpu_has_neon() or similar may be a good idea.

Cheers
---Dave