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

[Andy Lutomirski]

> On Nov 29, 2018, at 9:07 AM, Peter Zijlstra <pet...@infradead.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 <torva...@linux-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.