linux-kernel - Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

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Message-ID: <20181129170742.GC9027@hirez.programming.kicks-ass.net>
Date:   Thu, 29 Nov 2018 18:07:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, mhiramat@...nel.org,
        jbaron@...mai.com, Jiri Kosina <jkosina@...e.cz>,
        David.Laight@...lab.com, bp@...en8.de, julia@...com,
        jeyu@...nel.org, Peter Anvin <hpa@...or.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64

On Thu, Nov 29, 2018 at 09:02:23AM -0800, Andy Lutomirski wrote:
> > On Nov 29, 2018, at 8:50 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> > So no. Do *not* try to change %rsp on the stack in the bp handler.
> > Instead, I'd suggest:
> > 
> > - just restart the instruction (with the suggested "ptregs->rip --")
> > 
> > - to avoid any "oh, we're not making progress" issues, just fix the
> > instruction yourself to be the right call, by looking it up in the
> > "what needs to be fixed" tables.
> > 
> > No?

> Or do you think we can avoid the IPI while the int3 is there?

I'm thinking Linus is suggesting the #BP handler does the text write too
(as a competing store) and then sync_core() and restarts.

But I think that is broken, because then there is no telling what the
other CPUs will observe.