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

[Mathieu Desnoyers]

----- 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.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com