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

[Marc Zyngier]

On Mon, 22 Apr 2024 10:13:59 +0100,
Mark Rutland <mark.rutland at arm.com> wrote:
> 
> On Mon, Apr 22, 2024 at 02:04:43PM +0530, Naresh Kamboju wrote:
> > The arm64 defconfig build failed with gcc-8 and passed with gcc-13.
> > 
> > Reported-by: Linux Kernel Functional Testing <lkft at linaro.org>
> > 
> > Build log:
> > ---
> > /tmp/ccSUNNZy.s: Assembler messages:
> > /tmp/ccSUNNZy.s:3159: Error: unknown architectural extension `pauth'
> > make[5]: *** [scripts/Makefile.build:244: arch/arm64/kvm/pauth.o] Error 1
> > 
> > Steps to reproduce:
> > ---
> > # tuxmake --runtime podman --target-arch arm64 --toolchain gcc-8
> > --kconfig defconfig
> 
> I think the key thing here is GCC 8; the associated assembler won't necessarily
> have ARMv8.3-A support, since all the relevant bits got added around GCC 9.
> 
> Looking at the commits, I think this is broken since its introduction in commit:
> 
>   6ccc971ee2c61a1f  ("KVM: arm64: nv: Add emulation for ERETAx instructions")
> 
> ... where the pauth.c file only depends on ARM64_PTR_AUTH (which doesn't imply
> AS_HAS_ARMV8_3), but in the file we do:
> 
>   asm volatile(ARM64_ASM_PREAMBLE ".arch_extension pauth\n"
>                "pacga %0, %1, %2" : "=r" (pac) : "r" (ptr), "r" (mod));
> 
> 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.

This is what I've done [1]. It certainly isn't pretty, but this counts
as an incentive to drop some of the old stuff in the near-ish future.

I've pushed this out to -next, and hopefully the breakage will be
solved tomorrow.

	M.

[1] https://lore.kernel.org/r/20240422224849.2238222-1-maz@kernel.org

-- 
Without deviation from the norm, progress is not possible.