linux-kernel - Re: [patch V4 part 1 35/36] x86: Replace ist_enter() with nmi

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Date:   Fri, 8 May 2020 12:12:03 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, paulmck <paulmck@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>, rostedt <rostedt@...dmis.org>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>, Will Deacon <will@...nel.org>
Subject: Re: [patch V4 part 1 35/36] x86: Replace ist_enter() with nmi_enter()

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@...nel.org wrote:
> > 
> > > On Tue, May 5, 2020 at 7:14 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> > >>
> > >> From: Peter Zijlstra <peterz@...radead.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