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

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Fri, May 08, 2020 at 10:50:02AM +0200, Peter Zijlstra wrote:
> On Thu, May 07, 2020 at 02:17:58PM -0400, Mathieu Desnoyers wrote:
> > ----- On May 7, 2020, at 2:04 PM, Andy Lutomirski luto@kernel.org wrote:
> > 
> > > On Tue, May 5, 2020 at 7:14 AM Thomas Gleixner <tglx@linutronix.de> wrote:
> > >>
> > >> From: Peter Zijlstra <peterz@infradead.org>
> > >>
> > >> 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.

LALALALA

At least give a spoiler alert for those of us still enjoying part 1!

-- 
Josh