> -----Original Message-----
> From: Wilco Dijkstra <wilco.dijks...@arm.com>
> Sent: Friday, November 10, 2023 10:23 AM
> To: Kyrylo Tkachov <kyrylo.tkac...@arm.com>; GCC Patches <gcc-
> patc...@gcc.gnu.org>; Richard Sandiford <richard.sandif...@arm.com>
> Subject: Re: [PATCH] libatomic: Improve ifunc selection on AArch64
>
> Hi Kyrill,
>
> > + if (!(hwcap & HWCAP_CPUID))
> > + return false;
> > +
> > + unsigned long midr;
> > + asm volatile ("mrs %0, midr_el1" : "=r" (midr));
>
> > From what I recall that midr_el1 register is emulated by the kernel and so
> userspace software
> > has to check that the kernel supports that emulation through hwcaps before
> reading it.
> > According to https://www.kernel.org/doc/html/v5.8/arm64/cpu-feature-
> registers.html you
> > need to check getauxval(AT_HWCAP) & HWCAP_CPUID) before doing that
> read.
>
> That's why I do that immediately before reading midr_el1 - see above.
Errr, yes. Obviously I wasn't fully awake when I looked at it!
Sorry for the noise.
Ok for trunk then.
Kyrill
>
> Cheers,
> Wilco
