Messages in this thread | | | Date | Mon, 22 Apr 2024 14:11:05 +0200 | From | "Arnd Bergmann" <> | Subject | Re: gcc-8: arm64/kvm/pauth.: Error: unknown architectural extension `pauth' |
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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
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