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

From: Andy Lutomirski <luto@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Alexandre Chartre <alexandre.chartre@oracle.com>,
	Frederic Weisbecker <frederic@kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Petr Mladek <pmladek@suse.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Joel Fernandes <joel@joelfernandes.org>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Juergen Gross <jgross@suse.com>, Brian Gerst <brgerst@gmail.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Will Deacon <will@kernel.org>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Wei Liu <wei.liu@kernel.org>,
	Michael Kelley <mikelley@microsoft.com>,
	Jason Chen CJ <jason.cj.chen@intel.com>,
	Zhao Yakui <yakui.zhao@intel.com>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>
Subject: Re: [patch V6 07/37] x86/entry: Provide helpers for execute on irqstack
Date: Wed, 20 May 2020 08:36:57 -0700	[thread overview]
Message-ID: <CALCETrUTqiTjap72M3jENVOAs0m9G7tcbf++Mn-qiVmZXQ0eqA@mail.gmail.com> (raw)
In-Reply-To: <87d06ywrsf.fsf@nanos.tec.linutronix.de>

On Wed, May 20, 2020 at 8:27 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Andy Lutomirski <luto@kernel.org> writes:
> > On Wed, May 20, 2020 at 5:35 AM Thomas Gleixner <tglx@linutronix.de> wrote:
> >>
> >> Andy Lutomirski <luto@kernel.org> writes:
> >> > On Mon, May 18, 2020 at 4:53 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> >> >>
> >> >> Andy Lutomirski <luto@kernel.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.