linux-kernel - Re: [patch V6 07/37] x86/entry: Provide helpers for execute on irqstack

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Message-ID: <CALCETrUTqiTjap72M3jENVOAs0m9G7tcbf++Mn-qiVmZXQ0eqA@mail.gmail.com>
Date:   Wed, 20 May 2020 08:36:57 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "Paul E. McKenney" <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>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Wei Liu <wei.liu@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Jason Chen CJ <jason.cj.chen@...el.com>,
        Zhao Yakui <yakui.zhao@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [patch V6 07/37] x86/entry: Provide helpers for execute on irqstack

On Wed, May 20, 2020 at 8:27 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> Andy Lutomirski <luto@...nel.org> writes:
> > On Wed, May 20, 2020 at 5:35 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> >>
> >> Andy Lutomirski <luto@...nel.org> writes:
> >> > On Mon, May 18, 2020 at 4:53 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> >> >>
> >> >> Andy Lutomirski <luto@...nel.org> writes:
> >> >> > Actually, I revoke my ack.  Can you make one of two changes:
> >> >> >
> >> >> > Option A: Add an assertion to run_on_irqstack to verify that irq_count
> >> >> > was -1 at the beginning?  I suppose this also means you could just
> >> >> > explicitly write 0 instead of adding and subtracting.
> >> >> >
> >> >> > Option B: Make run_on_irqstack() just call the function on the current
> >> >> > stack if we're already on the irq stack.
> >> >> >
> >> >> > Right now, it's too easy to mess up and not verify the right
> >> >> > precondition before calling run_on_irqstack().
> >> >> >
> >> >> > If you choose A, perhaps add a helper to do the if(irq_needs_irqstack)
> >> >> > dance so that users can just do:
> >> >> >
> >> >> > run_on_irqstack_if_needed(...);
> >> >> >
> >> >> > instead of checking everything themselves.
> >> >>
> >> >> I'll have a look tomorrow morning with brain awake.
> >> >
> >> > Also, reading more of the series, I suspect that asm_call_on_stack is
> >> > logically in the wrong section or that the noinstr stuff is otherwise
> >> > not quite right.  I think that objtool should not accept
> >> > run_on_irqstack() from noinstr code.  See followups on patch 10.
> >>
> >> It's in entry.text which is non-instrumentable as well.
> >
> > Hmm.  I suppose we can chalk this up to the noinstr checking not being
> > entirely perfect.
>
> objtool considers both entry.text and noinstr.text. We just can't stick
> everything into entry.text for these !%@#45@# reasons.
>

Meh, this is all fine I think.  I think it would be slightly nicer if
objtool were to warn if noinstr code called run_on_stack(), but I'm
not sure it matters much.