* Dave Hansen <dave.han...@linux.intel.com> wrote:
> >> It's not a problem today because the instruction mask is always
> >> -1, so it always has every bit set that *MIGHT* be set in
> >> XSTATE_BV.
> >
> > Yes. And why would we ever want to narrow it?
>
> Because it actually allows us to take advantage of the compaction.
> Think of the layout of a task using protection keys and MPX.
>
> MPX = 8*4 + 8*2 = 48 bytes.
> PKEYs = 4 bytes
>
> They'll be spread out in the standard form *OR* the compacted form
> with a RFBM=-1. But, with the compacted form with RFBM=PK|MPX_BITS,
> they'll fit in a cacheline.
but but ... if this is a normal userspace task then it will use AVX on
modern Linux distros and then we are up to a 1K FPU context size
already, 48 bytes won't make a visible dent - especially if compacted
form has some saving cost. With AVX512 we are at around 2K?
I certainly don't _object_ to using the compacted format, as it makes
sense and it's clearly getting quite a bit of attention from the
hardware folks, but we should run some timings and such.
In any case I'm mostly just curious, not worried.
Thanks,
Ingo
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