Re: [patch V4 part 1 35/36] x86: Replace ist_enter() with nmi_enter()

[Peter Zijlstra]

On Thu, May 07, 2020 at 02:17:58PM -0400, Mathieu Desnoyers wrote:
> ----- On May 7, 2020, at 2:04 PM, Andy Lutomirski luto@xxxxxxxxxx wrote:
> 
> > On Tue, May 5, 2020 at 7:14 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> >>
> >> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >>
> >> 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. The interrupted context could be holding locks with interrupts
> >> disabled for instance.
> >>
> >> Similarly, #MC is an actual NMI-like exception.
> > 
> > Is it permissible to send a signal from inside nmi_enter()?  I imagine
> > so, but I just want to make sure.
> 
> If you mean sending a proper signal, I would guess not.
> 
> I suspect you'll rather want to use "irq_work()" from NMI context to ensure
> the rest of the work (e.g. sending a signal or a wakeup) is performed from
> IRQ context very soon after the NMI, rather than from NMI context.
> 
> AFAIK this is how this is done today by perf, ftrace, ebpf, and lttng.

What Mathieu says. But I suspect you want to keep reading until
part4-18. That should get you what you really want.