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

[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.