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

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Message-ID: <871rne6ayr.fsf@nanos.tec.linutronix.de>
Date:   Wed, 20 May 2020 14:35:08 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
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

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.