Re: Candidate Linux ABI for Intel AMX and hypothetical new related features

[Florian Weimer]

* Borislav Petkov:

> On Mon, Apr 12, 2021 at 04:19:29PM +0200, Florian Weimer wrote:
>> Maybe we could have done this in 2016 when I reported this for the first
>> time.  Now it is too late, as more and more software is using
>> CPUID-based detection for AVX-512.
>
> So as I said on another mail today, I don't think a library should rely
> solely on CPUID-based detection of features especially if those features
> need kernel support too. IOW, it should ask whether the kernel can
> handle those too, first.

Yes, that's why we have the XGETBV handshake.  I was imprecise.  It's
CPUID + XGETBV of course.  Or even AT_HWCAP2 (for FSGSBASE).

> And the CPUID-faulting thing would solve stuff like that because then
> the kernel can *actually* get involved into answering something where it
> has a say in, too.

But why wouldn't we use a syscall or an entry in the auxiliary vector
for that?  Why fault a potentially performance-critical instruction?

Thanks,
Florian