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

[Florian Weimer]

* Dave Hansen:

> On 5/21/21 7:44 AM, Florian Weimer wrote:
>> * Dave Hansen via Libc-alpha:
>>> Our system calls are *REALLY* fast.  We can even do a vsyscall for this
>>> if we want to get the overhead down near zero.  Userspace can also cache
>>> the "I did the prctl()" state in thread-local storage if it wants to
>>> avoid the syscall.
>> Why can't userspace look at XCR0 to make the decision?
>
> The thing we're trying to avoid is a #NM exception from XFD (the new
> first-use detection feature) that occurs on the first use of AMX.
> XCR0 will have XCR0[AMX]=1, even if XFD is "armed" and ready to
> generate the #NM.

I see.  So essentially the hardware wants to offer transparent
initialize-on-use, but Linux does not seem to want to implement it this
way.

Is there still a chance to bring the hardware and Linux into alignment?

Thanks,
Florian