linux-kernel - Re: gcc-8: arm64/kvm/pauth.: Error: unknown architectural extension `pauth'

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Message-Id: <df782e40-95bf-4f8e-96c8-355ff0109e3a@app.fastmail.com>
Date: Mon, 22 Apr 2024 14:11:05 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Mark Rutland" <mark.rutland@....com>
Cc: "Naresh Kamboju" <naresh.kamboju@...aro.org>,
 "open list" <linux-kernel@...r.kernel.org>,
 "Linux ARM" <linux-arm-kernel@...ts.infradead.org>,
 lkft-triage@...ts.linaro.org,
 "Linux Regressions" <regressions@...ts.linux.dev>,
 "Anders Roxell" <anders.roxell@...aro.org>, "Marc Zyngier" <maz@...nel.org>,
 joey.gouly@....com, "Oliver Upton" <oliver.upton@...ux.dev>
Subject: Re: gcc-8: arm64/kvm/pauth.: Error: unknown architectural extension `pauth'

On Mon, Apr 22, 2024, at 11:40, Mark Rutland wrote:
> On Mon, Apr 22, 2024 at 11:25:25AM +0200, Arnd Bergmann wrote:
>> On Mon, Apr 22, 2024, at 11:13, Mark Rutland wrote:
>> > On Mon, Apr 22, 2024 at 02:04:43PM +0530, Naresh Kamboju wrote:
>> > Given the minimum supported toolchain comes with an assembler that doesn't
>> > necessarily support ARMv8.3, I reckon we'll either have to make NV pauth
>> > support depend upon AS_HAS_ARMV8_3, or manually assemble the PACGA instruction.
>> >
>> > I suspect the latter is the better option.
>> 
>> The .config linked from the report shows
>> 
>> CONFIG_AS_VERSION=23101
>> CONFIG_ARM64_PTR_AUTH_KERNEL=y
>> CONFIG_AS_HAS_ARMV8_3=y
>> 
>> So it gets detected as supporting ARMv8.3. Is this the wrong
>> conditional to check, or does it get misdetected for an unsupported
>> assembler?
>
> I suspect that means the 'pauth' arch extension was added after armv8.3
> support, and the assembler supports `-march=armv8.3-a` but does not support
> `.arch_extension pauth`. So for this code, it'd be wrong to check for
> AS_HAS_ARMV8_3, unless we used `.march armv8.3-a`, but even then that'd still
> mean configurations where we couldn't support this code.
>
> I reckon manually assembing the PACGA is the best thing to do; that sidesteps
> the need for either `.arch_extension pauth` or `.march armv8.3-a`, and aligns
> with what we do for CONFIG_ARM64_PTR_AUTH=y generally.
>
> Elsewhere in the kernel where we check for CONFIG_AS_HAS_ARMV8_3, we rely on
> ARM64_ASM_PREAMBLE containing `.arch armv8.3-a` or a later version that implies
> the presence of ARMv8.3-A instructions, and so pauth usage elsewhere is fine.

I tested with the old binutils versions I have here and found
that anything that supports v8.3 also understands pacga, but
'.arch_extension pauth' only works in binutils-2.35 and higher,
presumably because it started out as a v8.3+ feature but was
later turned into an optional extension for all versions.

Since there is a Kconfig check for armv8.3-a support already, I think
it's safe to just drop the .arch_extension pauth.

      Arnd