Re: FPU register granularity [Was: Re: [PATCH crypto-stable] crypto: arch/lib - limit simd usage to PAGE_SIZE chunks]

[Jason A. Donenfeld]

On Mon, Apr 20, 2020 at 10:14 PM Jason A. Donenfeld <Jason@xxxxxxxxx> wrote:
>
> Hi David,
>
> On Mon, Apr 20, 2020 at 2:32 AM David Laight <David.Laight@xxxxxxxxxx> wrote:
> > Maybe kernel_fp_begin() should be passed the address of somewhere
> > the address of an fpu save area buffer can be written to.
> > Then the pre-emption code can allocate the buffer and save the
> > state into it.
>
> Interesting idea. It looks like `struct xregs_state` is only 576
> bytes. That's not exactly small, but it's not insanely huge either,
> and maybe we could justifiably stick that on the stack, or even
> reserve part of the stack allocation for that that the function would
> know about, without needing to specify any address.

Hah-hah, nevermind here. extended_state_area is of course huge,
bringing the whole structure to a whopping 3k with avx512. :)