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

[Andy Lutomirski]

Sigh, cc linux-api, not linux-abi.

On Fri, Mar 26, 2021 at 4:12 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
> Hi all-
>
> After some discussion on IRC, I have a proposal for a Linux ABI for
> using Intel AMX and other similar features.  It works like this:
>
> First, we make XCR0 dynamic.  This looks a lot like Keno's patch but
> with a different API, outlined below.  Different tasks can have
> different XCR0 values.  The default XCR0 for new tasks does not
> include big features like AMX.  XMM and YMM are still there.  The AVX2
> states are debatable -- see below.
>
> To detect features and control XCR0, we add some new arch_prctls:
>
> arch_prctl(ARCH_GET_XCR0_SUPPORT, 0, ...);
>
> returns the set of XCR0 bits supported on the current kernel.
>
> arch_prctl(ARCH_GET_XCR0_LAZY_SUPPORT, 0, ...);
>
> returns 0.  See below.
>
> arch_prctl(ARCH_SET_XCR0, xcr0, lazy_states, sigsave_states,
> sigclear_states, 0);
>
> Sets xcr0.  All states are preallocated except that states in
> lazy_states may be unallocated in the kernel until used.  (Not
> supported at all in v1.  lazy_states & ~xcr0 != 0 is illegal.)  States
> in sigsave_states are saved in the signal frame.  States in
> sigclear_states are reset to the init state on signal delivery.
> States in sigsave_states are restored by sigreturn, and states not in
> sigsave_states are left alone by sigreturn.
>
> Optionally we do not support PKRU at all in XCR0 -- it doesn't make
> that much sense as an XSAVE feature, and I'm not convinced that trying
> to correctly context switch XINUSE[PKRU] is worthwhile.  I doubt we
> get it right today.
>
> Optionally we come up with a new format for new features in the signal
> frame, since the current format is showing its age.  Taking 8kB for a
> signal with AMX is one thing.  Taking another 8kB for a nested signal
> if AMX is not in use is worse.
>
> Optionally we make AVX-512 also default off, which fixes what is
> arguably a serious ABI break with AVX-512: lots of programs, following
> POSIX (!), seem to think that they know much much space to allocate
> for sigaltstack().   AVX-512 is too big.
>
> Thoughts?
>
> --Andy