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

[Dave Hansen]

On 1/15/19 12:26 PM, Andy Lutomirski wrote:
> 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.

Using AVX-512 as an example...

Let's say there was AVX-512 state, and a kernel_fpu_begin() user only
used AVX2.  We could totally avoid doing *any* AVX-512 state save/restore.

The init optimization doesn't help us if there _is_ AVX-512 state, and
the modified optimization only helps if we recently did a XRSTOR at
context switch and have not written to AVX-512 state since XRSTOR.

This probably only matters for AVX-512-using apps that have run on a
kernel with lots of kernel_fpu_begin()s that don't use AVX-512.  So, not
a big deal right now.