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

[Len Brown]

On Mon, Aug 30, 2021 at 1:52 PM Borislav Petkov <bp@xxxxxxxxx> 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