Re: [PATCH v4 05/27] x86: Replace ist_enter() with nmi_enter()

[Andy Lutomirski]

On Fri, Feb 21, 2020 at 5:50 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> A few exceptions (like #DB and #BP) can happen at any location in the
> code, this then means that tracers should treat events from these
> exceptions as NMI-like. We could be holding locks with interrupts
> disabled for instance.
>
> Similarly, #MC is an actual NMI-like exception.
>

> -dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
> +dotraplinkage notrace void do_int3(struct pt_regs *regs, long error_code)
>  {
>         if (poke_int3_handler(regs))
>                 return;
>
> -       /*
> -        * Use ist_enter despite the fact that we don't use an IST stack.
> -        * We can be called from a kprobe in non-CONTEXT_KERNEL kernel
> -        * mode or even during context tracking state changes.
> -        *
> -        * This means that we can't schedule.  That's okay.
> -        */
> -       ist_enter(regs);
> +       nmi_enter();

I agree with the change, but some commentary might be nice.  Maybe
copy from here:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/idtentry&id=061eaa900b4f63601ab6381ab431fcef8dfd84be