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

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Message-Id: <6C359EF7-B2B6-4FED-96EB-CB7C91918763@amacapital.net>
Date:   Thu, 29 Nov 2018 09:31:04 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
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 Nov 29, 2018, at 9:07 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> 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.

Does anyone know what the actual hardware semantics are?  The SDM is not particularly informative unless I looked at the wrong section.