Re: [PATCH 1/1] headers/deps: x86/fpu: Make task_struct::thread constant size

Oleg Nesterov
2024-03-26T21:56:27Z

On 03/26, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@redhat.com> wrote:
>
> > > +static struct fpu x86_init_fpu __read_mostly;
> > > +
> > > static void __init fpu__init_system_early_generic(void)
> > > {
> > > + {
> > > + int this_cpu = smp_processor_id();
> > > +
> > > + fpstate_reset(&x86_init_fpu);
> > > + current->thread.fpu = &x86_init_fpu;
> > > + per_cpu(fpu_fpregs_owner_ctx, this_cpu) = &x86_init_fpu;
> > > + x86_init_fpu.last_cpu = this_cpu;
> > > + }
> >
> > Can't x86_init_fpu be declared inside the block above?
>
> As a function-local static? I think globals are better defined in a visible
> fashion, not hidden among local variables where they are easy to overlook.

OK, but please see below.

> Yeah. Something like the delta patch below?

LGTM. And the whole patch too, although I don't think I can review it ;)

but... Ingo, let me ask 2 stupid questions.

1. Can you split this patch? I mean, 1/2 should add something like

struct fpu *x86_task_fpu(struct task_struct *task)
{
return &target->thread.fpu;
}

somewhere in arch/x86/include and update all the users of ->thread.fpu
to use the new helper. No functional changes.

This way 2/2 which actually turns .fpu into a pointer will be MUCH simpler.

2. Now, 2/2 should change the new helper above to

return target->thread.fpu;

But! why do we need the thread_struct.fpu pointer at all? Can't we simply have

struct fpu *x86_task_fpu(struct task_struct *task)
{
return (void *)task + sizeof(*task);
}

? This is what fpu_clone() (with your patch) does anyway.

Yes, this needs some changes:

- exit_thread() - trivial

- arch_dup_task_struct() clears thread.fpu, but I guess this is unnecessary

- fpu_clone() initializes dst->thread.fpu, no longer needed

- finally, fpu__init_system_early_generic() which initializes the
init_task.thread.fpu pointer.

But this doesn't look difficult? Although I don't understand the magic in
arch/x86/kernel/vmlinux.lds.S ...

Oleg.