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

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Message-ID: <87bl9s8kfb.fsf@oldenburg.str.redhat.com>
Date:   Mon, 03 May 2021 07:18:16 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Len Brown <lenb@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>, Willy Tarreau <w@....eu>,
        Andy Lutomirski <luto@...nel.org>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        Dave Hansen <dave.hansen@...el.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-abi@...r.kernel.org,
        "libc-alpha@...rceware.org" <libc-alpha@...rceware.org>,
        Rich Felker <dalias@...c.org>, Kyle Huey <me@...ehuey.com>,
        Keno Fischer <keno@...iacomputing.com>
Subject: Re: Candidate Linux ABI for Intel AMX and hypothetical new related
 features

* Len Brown:

> 5. If the feature is enabled in XCR0, the user happily uses it.
>
>     For AMX, Linux implements "transparent first use"
>     so that it doesn't have to allocate 8KB context switch
>     buffers for tasks that don't actually use AMX.
>     It does this by arming XFD for all tasks, and taking a #NM
>     to allocate a context switch buffer only for those tasks
>     that actually execute AMX instructions.

What happens if the kernel cannot allocate that additional context
switch buffer?

Thanks,
Florian