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Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20181129170742.GC9027@hirez.programming.kicks-ass.net>
Date:   Thu, 29 Nov 2018 09:31:04 -0800
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Andrew Lutomirski <luto@kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, mhiramat@kernel.org,
        jbaron@akamai.com, Jiri Kosina <jkosina@suse.cz>,
        David.Laight@aculab.com, bp@alien8.de, julia@ni.com,
        jeyu@kernel.org, Peter Anvin <hpa@zytor.com>
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To:     Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Nov 29, 2018, at 9:07 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>=20
> On Thu, Nov 29, 2018 at 09:02:23AM -0800, Andy Lutomirski wrote:
>>> On Nov 29, 2018, at 8:50 AM, Linus Torvalds <torvalds@linux-foundation.o=
rg> wrote:
>=20
>>> So no. Do *not* try to change %rsp on the stack in the bp handler.
>>> Instead, I'd suggest:
>>>=20
>>> - just restart the instruction (with the suggested "ptregs->rip --")
>>>=20
>>> - 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.
>>>=20
>>> No?
>=20
>> Or do you think we can avoid the IPI while the int3 is there?
>=20
> I'm thinking Linus is suggesting the #BP handler does the text write too
> (as a competing store) and then sync_core() and restarts.
>=20
> 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 par=
ticularly informative unless I looked at the wrong section.=