On Sat, Sep 1, 2018 at 1:19 PM, Jason A. Donenfeld [off-list ref] wrote:

Hey Thomas,

I'd like to move ahead with my patchset and make some forward progress
in LKML submission. If you've got something brewing regarding the FPU
context on x86 and ARM, I'm happy to wait a bit longer so as to build
on that. But if that is instead a far-off theoretical eventual thing,
perhaps it's better for me to move ahead as planned, and we can switch
to the superior FPU semantics whenever you get around to it? Either
way, please let me know what you have in mind so our plans can stay
somewhat sync'd.

I tend to think the right approach is to merge Jason's code and then
make it better later.  Even with a totally perfect lazy FPU restore
implementation on x86, we'll probably still need some way of dealing
with SIMD contexts.  I think we're highly unlikely to ever a allow
SIMD usage in all NMI contexts, for example, and there will always be
cases where we specifically don't want to use all available SIMD
capabilities even if we can.  For example, generating random numbers
does crypto, but we probably don't want to do *SIMD* crypto, since
that will force a save and restore and will probably fire up the
AVX512 unit, and that's not worth it unless we're already using it for
some other reason.

Also, as Rik has discovered, lazy FPU restore is conceptually
straightforward but isn't entirely trivial :)

--Andy