* Dave Hansen <dave.hansen@xxxxxxxxx> wrote: > On 3/5/25 09:37, Ingo Molnar wrote: > > > > * Dave Hansen <dave.hansen@xxxxxxxxx> wrote: > > > >> On 3/5/25 01:07, Ingo Molnar wrote:>> Alternatives considered: > >>>> - Make kernel-mode FPU sections fully preemptible. This would require > >>>> growing task_struct by another struct fpstate which is more than 2K. > >>> > >>> So that's something that will probably happen once the kernel is built > >>> using APX anyway? > >> > >> I was expecting that building the kernel with APX would be very > >> different than a kernel_fpu_begin(). We don't just need *one* more > >> save area for APX registers: we need a stack, just like normal GPRs. > > > > Yes - but my point is: with any APX build we'd probably be saving > > FPU(-ish) registers at entry points, into a separate context area. If > > that includes FPU registers then we'd not have to do > > kernel_fpu_begin()/end(). > > That's true. But wouldn't it be a bit silly to include _all_ FPU > registers? If the kernel isn't using AVX512, why bother saving and > restoring AVX512? Fair enough - although I bet the execution time difference between a partial and a full FPU context save isn't as large as the buffer size would suggest... There's a lot of setup cost in XSAVE* instructions last I checked. Thanks, Ingo
