linux-kernel - Re: [PATCH v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to protect dynamic user state

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Date:   Tue, 31 Aug 2021 18:07:23 -0400
From:   Len Brown <lenb@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     "Chang S. Bae" <chang.seok.bae@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Thiago Macieira <thiago.macieira@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to
 protect dynamic user state

On Mon, Aug 30, 2021 at 1:52 PM Borislav Petkov <bp@...en8.de> wrote:

> Well, if you preallocate everything...

Nothing prevents, say, a pthread_create() or anything
else where the kernel consumes memory on behalf of a process
from failing at run-time...  AMX does not add a unique OOM risk here.

> > The advantage of the #NM over the syscall is that the programmer
> > doesn't actually have to do anything. Also, transparently allocated
> > buffers offer a theoretical benefit that a program may have many
> > threads, but only a few may actually touch AMX, and so there is
> > savings to be had by allocating buffers only for the threads that
> > actually use the buffers.
>
> The program already asked the kernel whether it can use AMX - it can
> allocate the buffers for the threads too.

The result is that if one thread in a 1,000 task process requests
and touches AMX, the kernel would allocate 8MB, instead of 8KB
of context switch buffers for that process, no?

Len Brown, Intel Open Source Technology Center