Re: [Patch v5 06/19] perf/x86: Add support for XMM registers in non-PEBS and REGS_USER

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Dec 04, 2025 at 10:59:15AM -0800, Dave Hansen wrote:
> On 12/4/25 07:17, Peter Zijlstra wrote:
> >> - Additionally, checking the TIF_NEED_FPU_LOAD flag alone is insufficient.
> >>   Some corner cases, such as an NMI occurring just after the flag switches
> >>   but still in kernel mode, cannot be handled.
> > Urgh.. Dave, Thomas, is there any reason we could not set
> > TIF_NEED_FPU_LOAD *after* doing the XSAVE (clearing is already done
> > after restore).
> > 
> > That way, when an NMI sees TIF_NEED_FPU_LOAD it knows the task copy is
> > consistent.
> 
> Something like the attached patch?
> 
> I think that would be just fine. save_fpregs_to_fpstate() doesn't
> actually change the need for TIF_NEED_FPU_LOAD, so I don't think the
> ordering matters.

Right, I missed this one. And yes, I couldn't find any site where this
ordering mattered either. Its all with interrupts disabled, so normally
it all goes together. Only the NMI could observe the difference.

> diff --git a/arch/x86/include/asm/fpu/sched.h b/arch/x86/include/asm/fpu/sched.h
> index 89004f4ca208..2d57a7bf5406 100644
> --- a/arch/x86/include/asm/fpu/sched.h
> +++ b/arch/x86/include/asm/fpu/sched.h
> @@ -36,8 +36,8 @@ static inline void switch_fpu(struct task_struct *old, int cpu)
>  	    !(old->flags & (PF_KTHREAD | PF_USER_WORKER))) {
>  		struct fpu *old_fpu = x86_task_fpu(old);
>  
> -		set_tsk_thread_flag(old, TIF_NEED_FPU_LOAD);
>  		save_fpregs_to_fpstate(old_fpu);
> +		set_tsk_thread_flag(old, TIF_NEED_FPU_LOAD);
>  		/*
>  		 * The save operation preserved register state, so the
>  		 * fpu_fpregs_owner_ctx is still @old_fpu. Store the