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

[Jason A. Donenfeld]

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.

> kernel_fpu_begin() ought also be passed a parameter saying which
> fpu features are required, and return which are allocated.
> On x86 this could be used to check for AVX512 (etc) which may be
> available in an ISR unless it interrupted inside a kernel_fpu_begin()
> section (etc).
> It would also allow optimisations if only 1 or 2 fpu registers are
> needed (eg for some of the crypto functions) rather than the whole
> fpu register set.

For AVX512 this probably makes sense, I suppose. But I'm not sure if
there are too many bits of crypto code that only use a few registers.
There are those accelerated memcpy routines in i915 though -- ever see
drivers/gpu/drm/i915/i915_memcpy.c? sort of wild. But if we did go
this way, I wonder if it'd make sense to totally overengineer it and
write a gcc/as plugin to create the register mask for us. Or, maybe
some checker inside of objtool could help here.

Actually, though, the thing I've been wondering about is actually
moving in the complete opposite direction: is there some
efficient-enough way that we could allow FPU registers in all contexts
always, without the need for kernel_fpu_begin/end? I was reversing
ntoskrnl.exe and was kind of impressed (maybe not the right word?) by
their judicious use of vectorisation everywhere. I assume a lot of
that is being generated by their compiler, which of course gcc could
do for us if we let it. Is that an interesting avenue to consider? Or
are you pretty certain that it'd be a huge mistake, with an
irreversible speed hit?

Jason