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

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Date:   Fri, 21 May 2021 23:43:38 +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 13:07, Andy Lutomirski wrote:
> On Fri, May 21, 2021, at 12:10 PM, Thomas Gleixner wrote:
>> 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.
>
> XFD does nothing about signals.

It's a matter of what's implemented in #NM. XFD just arms #NM

> It also doesn’t help give applications a non-Linux-specific way to ask
> if AMX is available. The SDM says that one can read XCR0.  Sure, we
> can use it, but cross platform libraries seem likely to get it wrong.

Well, that's the inevitable consequence of Intel declaring that
everything needs to be exposed unconditionally for the very wrong
reasons.

Thanks,

        tglx