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

On 8/31/21 3:07 PM, Len Brown wrote:

On Mon, Aug 30, 2021 at 1:52 PM Borislav Petkov [off-list ref] wrote:

quoted

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.

quoted

quoted

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?

Yes, but that's a pretty natural consequence of the process-level ABI
which was chosen.  A per-thread permission scheme would not have had
this particular trade-off.

If you have a big process (lots of threads) and you use a process-level
ABI, there are going to big implications.  I don't think we can get away
from this.