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

[Ingo Molnar]

* 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().

In other words, we'd be doing something close to 'growing task_struct 
by another struct fpstate', or so - regardless of whether it's in 
task_struct or some sort of extended pt_regs. The kernel would also be 
close to 'FPU-safe', i.e. there likely wouldn't be a need for 
kernel_fpu_begin()/end().

Thanks,

	Ingo