Re: [PATCH 4/8] x86/fpu: Remove the thread::fpu pointer

[Oleg Nesterov <oleg@redhat.com>]

On 04/10, Ingo Molnar wrote:
>
> * Peter Zijlstra <peterz@infradead.org> wrote:
>
> > On Wed, Apr 09, 2025 at 11:11:23PM +0200, Ingo Molnar wrote:
> >
> > > -#define x86_task_fpu(task) ((task)->thread.fpu)
> > > +#define x86_task_fpu(task)	((struct fpu *)((void *)(task) + sizeof(*(task))))
> >
> > Doesn't our FPU state need to be cacheline aligned?
>
> Yeah, and we do have a check for that:
>
> +       BUILD_BUG_ON(sizeof(*dst) % SMP_CACHE_BYTES != 0);
>
> And task_struct is allocated cache-aligned, which means when we do this
> in fpu_clone():
>
> +       struct fpu *dst_fpu = (void *)dst + sizeof(*dst);
>
> the FPU pointer is guaranteed to be cacheline aligned as well.
>
> 'dst' in that context is the new task_struct.
>
> BTW., Oleg suggested in a previous discussion for us to replace the
> task->thread.fpu pointer with a build-time calculation - but I'm still
> not sure it's a good idea.

To be honest, I forgot everything we discussed before ;)

But I have found this email,
https://lore.kernel.org/all/20240616105550.GA18292@redhat.com/
Perhaps

	#define X86_TASK_SIZE	\
		ALIGN(sizeof(struct task_struct), __alignof__(union fpregs_state))

and
	#define x86_task_fpu(task)	\
		((struct fpu *)((void *)(task) + X86_TASK_SIZE))

plus a bit more similar changes make more sense than
__attribute__ ((aligned (64))) for struct task_struct?

OK, even if yes, we can do this later on top of this series.

I'll try to read it tomorrow.

Oleg.