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

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Date:   Fri, 21 May 2021 21:10:29 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     Dave Hansen via Libc-alpha <libc-alpha@...rceware.org>,
        Len Brown <lenb@...nel.org>, Rich Felker <dalias@...c.org>,
        Linux API <linux-api@...r.kernel.org>,
        "Bae\, Chang Seok" <chang.seok.bae@...el.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kyle Huey <me@...ehuey.com>, Borislav Petkov <bp@...en8.de>,
        Keno Fischer <keno@...iacomputing.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Willy Tarreau <w@....eu>
Subject: Re: Candidate Linux ABI for Intel AMX and hypothetical new related features

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’t have a huge problem with
> it.

I'm fine with that. It's still controlled by the OS and can return
-EPERM.

If allowed then the application would also accept to be insta killed if
that #NM allocation fails. Any bug report vs. that will be ignored.

> I think several things here are regrettable:
>
> 1. Legacy XSTATE code might assume that XCR0 is a constant.
>
> 2. Intel virt really doesn’t like us context switching XCR0, although
> we might say that this is Intel’s fault and therefore Intel’s
> problem. AMD hardware doesn’t 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.

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.

Thanks,

        tglx