Messages in this thread | | | Subject | Re: [PATCH v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to protect dynamic user state | From | Dave Hansen <> | Date | Tue, 31 Aug 2021 15:11:59 -0700 |
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On 8/31/21 3:07 PM, Len Brown wrote: > On Mon, Aug 30, 2021 at 1:52 PM Borislav Petkov <bp@alien8.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?
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.
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