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

[Andy Lutomirski]

> On Apr 12, 2021, at 7:38 AM, Florian Weimer <fweimer@xxxxxxxxxx> wrote:
> 
> * 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?
> 

CPUID is horrifically slow in various virt scenarios. If user code needs to serialize, use IRET or SERIALIZE.