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.
