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

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Message-ID: <ZiYwkKLRVgvrQrnK@FVFF77S0Q05N>
Date: Mon, 22 Apr 2024 10:40:32 +0100
From: Mark Rutland <mark.rutland@....com>
To: Arnd Bergmann <arnd@...db.de>
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 11:25:25AM +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.

Mark.