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Date:   Fri, 21 May 2021 13:07:52 -0700
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "Thomas Gleixner" <tglx@linutronix.de>,
        "Florian Weimer" <fweimer@redhat.com>,
        "Dave Hansen" <dave.hansen@intel.com>
Cc:     "Dave Hansen via Libc-alpha" <libc-alpha@sourceware.org>,
        "Len Brown" <lenb@kernel.org>, "Rich Felker" <dalias@libc.org>,
        "Linux API" <linux-api@vger.kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
        "Kyle Huey" <me@kylehuey.com>, "Borislav Petkov" <bp@alien8.de>,
        "Keno Fischer" <keno@juliacomputing.com>,
        "Arjan van de Ven" <arjan@linux.intel.com>,
        "Willy Tarreau" <w@1wt.eu>
Subject: =?UTF-8?Q?Re:_Candidate_Linux_ABI_for_Intel_AMX_and_hypothetical_new_rel?=
 =?UTF-8?Q?ated_features?=
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On Fri, May 21, 2021, at 12:10 PM, Thomas Gleixner wrote:
> On Fri, May 21 2021 at 09:31, Andy Lutomirski wrote:
> > arch_prctl(SET_XSTATE_INIT_ON_FIRST_USE, TILE_STUFF);?
> >
> > As long as this is allowed to fail, I don=E2=80=99t have a huge prob=
lem with
> > it.
>=20
> I'm fine with that. It's still controlled by the OS and can return
> -EPERM.
>=20
> If allowed then the application would also accept to be insta killed i=
f
> that #NM allocation fails. Any bug report vs. that will be ignored.
>=20
> > I think several things here are regrettable:
> >
> > 1. Legacy XSTATE code might assume that XCR0 is a constant.
> >
> > 2. Intel virt really doesn=E2=80=99t like us context switching XCR0,=
 although
> > we might say that this is Intel=E2=80=99s fault and therefore Intel=E2=
=80=99s
> > problem. AMD hardware doesn=E2=80=99t appear to have this issue.
> >
> > 3. AMX bring tangled up in XSTATE is unfortunate.  The whole XSTATE
> > mechanism is less than amazing.
> >
> > IMO the best we can make of this whole situation is to make XCR0
> > dynamic, but the legacy compatibility issues are potentially
> > problematic.
>=20
> Why? The bit can be enabled and #NM catches the violation of the ABI
> contract if the application did not request usage. No XCR0 fiddling on=

> context switch required.
>=20
> Thanks,
>=20
>         tglx
>=20
>=20
>=20

XFD does nothing about signals.  It also doesn=E2=80=99t help give appli=
cations a non-Linux-specific way to ask if AMX is available. The SDM say=
s that one can read XCR0.  Sure, we can use it, but cross platform libra=
ries seem likely to get it wrong.