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

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Date:   Sat, 08 May 2021 11:45:51 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Len Brown <lenb@...nel.org>, Borislav Petkov <bp@...en8.de>
Cc:     Willy Tarreau <w@....eu>, Andy Lutomirski <luto@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        "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\@sourceware.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

On Sun, May 02 2021 at 11:27, Len Brown wrote:
> On Fri, Apr 23, 2021 at 3:58 PM Borislav Petkov <bp@...en8.de> wrote:
> Here is how it works:
>
> 1. The kernel boots and sees the feature in CPUID.
>
> 2. If the kernel supports that feature, it sets XCR0[feature].
>
>     For some features, there may be a bunch of kernel support,
>     while simple features may require only state save/restore.
>
> 2a.  If the kernel doesn't support the feature, XCR0[feature] remains cleared.
>
> 3. user-space sees the feature in CPUID
>
> 4. user-space sees for the feature via xgetbv[XCR0]
>
> 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.
>
> 5a. If the feature is not enabled in XCR0, and the tasks uses
>     those instructions anway, the hardware delivers a #UD
>     and the kernel kills the process with a signal.

Where is #6 which describes the signal interaction?

Thanks,

        tglx