Re: [Patch v6 08/22] x86/fpu: Ensure TIF_NEED_FPU_LOAD is set after saving FPU state

[Peter Zijlstra]

On Wed, Feb 11, 2026 at 11:55:10AM -0800, Dave Hansen wrote:

> This:
> 
> >  	/* Swap fpstate */
> >  	if (enter_guest) {
> > -		fpu->__task_fpstate = cur_fps;
> > +		WRITE_ONCE(fpu->__task_fpstate, cur_fps);
> > +		barrier();
> >  		fpu->fpstate = guest_fps;
> >  		guest_fps->in_use = true;
> >  	} else {
> >  		guest_fps->in_use = false;
> >  		fpu->fpstate = fpu->__task_fpstate;
> > -		fpu->__task_fpstate = NULL;
> > +		barrier();
> > +		WRITE_ONCE(fpu->__task_fpstate, NULL);
> >  	}
> 
> also urgently needs comments.
> 
> I also can't help but think that there might be a nicer way to do that
> without the barrier(). I _think_ two correctly-ordered WRITE_ONCE()'s
> would make the compiler do the same thing as the barrier().
> 
> But I'm not fully understanding what the barrier() is doing anyway, so
> take that with a grain of salt.

barrier() restricts load/store movement by the compiler. A load/store
cannot be moved across barrier().