Re: [RFC PATCH v2] x86/fpu: make kernel-mode FPU reliably usable in softirqs

[Ingo Molnar <mingo@kernel.org>]

* Dave Hansen <dave.hansen@intel.com> wrote:

> On 3/5/25 09:37, Ingo Molnar wrote:
> > 
> > * Dave Hansen <dave.hansen@intel.com> 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