Re: [PATCH v6] x86: load FPU registers on return to userland

[Andy Lutomirski <luto@kernel.org>]

On Tue, Jan 15, 2019 at 11:46 AM Dave Hansen <dave.hansen@intel.com> wrote:
>
> On 1/15/19 4:44 AM, David Laight wrote:
> > Once this is done it might be worth while adding a parameter to
> > kernel_fpu_begin() to request the registers only when they don't
> > need saving.
> > This would benefit code paths where the gains are reasonable but not massive.
> >
> > The return value from kernel_fpu_begin() ought to indicate which
> > registers are available - none, SSE, SSE2, AVX, AVX512 etc.
> > So code can use an appropriate implementation.
> > (I've not looked to see if this is already the case!)
>
> Yeah, it would be sane to have both a mask passed, and returned, say:
>
>         got = kernel_fpu_begin(XFEATURE_MASK_AVX512, NO_XSAVE_ALLOWED);
>
>         if (got == XFEATURE_MASK_AVX512)
>                 do_avx_512_goo();
>         else
>                 do_integer_goo();
>
>         kernel_fpu_end(got)
>
> Then, kernel_fpu_begin() can actually work without even *doing* an XSAVE:
>
>         /* Do we have to save state for anything in 'ask_mask'? */
>         if (all_states_are_init(ask_mask))
>                 return ask_mask;
>
> Then kernel_fpu_end() just needs to zero out (re-init) the state, which
> it can do with XRSTORS and a careful combination of XSTATE_BV and the
> requested feature bitmap (RFBM).
>
> This is all just optimization, though.

I don't think we'd ever want kernel_fpu_end() to restore anything,
right?  I'm a bit confused as to when this optimization would actually
be useful.

Jason Donenfeld has a rather nice API for this in his Zinc series.
Jason, how is that coming?