Re: [PATCH v3 0/6] Static calls

[Josh Poimboeuf]

On Thu, Jan 10, 2019 at 08:48:31PM +0000, Nadav Amit wrote:
> > On Jan 10, 2019, at 12:32 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > 
> > On Thu, Jan 10, 2019 at 07:45:26PM +0000, Nadav Amit wrote:
> >>>> Iâm not GCC expert either and writing this code was not making me full of
> >>>> joy, etc.. Iâll be happy that my code would be reviewed, but it does work. I
> >>>> donât think an early pass is needed, as long as hardware registers were not
> >>>> allocated.
> >>>> 
> >>>>> Would it work with more than 5 arguments, where args get passed on the
> >>>>> stack?
> >>>> 
> >>>> It does.
> >>>> 
> >>>>> At the very least, it would (at least partially) defeat the point of the
> >>>>> callee-saved paravirt ops.
> >>>> 
> >>>> Actually, I think you can even deal with callee-saved functions and remove
> >>>> all the (terrible) macros. You would need to tell the extension not to
> >>>> clobber the registers through a new attribute.
> >>> 
> >>> Ok, it does sound interesting then.  I assume you'll be sharing the
> >>> code?
> >> 
> >> Of course. If this what is going to convince, Iâll make a small version for
> >> PV callee-saved first.
> > 
> > It wasn't *only* the PV callee-saved part which interested me, so if you
> > already have something which implements the other parts, I'd still like
> > to see it.
> 
> Did you have a look at https://lore.kernel.org/lkml/20181231072112.21051-4-namit@xxxxxxxxxx/ ?
> 
> See the changes to x86_call_markup_plugin.c .
> 
> The missing part (that I just finished but need to cleanup) is attributes
> that allow you to provide key and dynamically enable the patching.

Aha, so it's the basically the same plugin you had for optpolines.  I
missed that.  I'll need to stare at the code for a little bit.

> >>>>> What if we just used a plugin in a simpler fashion -- to do call site
> >>>>> alignment, if necessary, to ensure the instruction doesn't cross
> >>>>> cacheline boundaries.  This could be done in a later pass, with no side
> >>>>> effects other than code layout.  And it would allow us to avoid
> >>>>> breakpoints altogether -- again, assuming somebody can verify that
> >>>>> intra-cacheline call destination writes are atomic with respect to
> >>>>> instruction decoder reads.
> >>>> 
> >>>> The plugin should not be able to do so. Layout of the bytecode is done by
> >>>> the assembler, so I donât think a plugin would help you with this one.
> >>> 
> >>> Actually I think we could use .bundle_align_mode for this purpose:
> >>> 
> >>> https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fsourceware.org%2Fbinutils%2Fdocs-2.31%2Fas%2FBundle-directives.html&amp;data=02%7C01%7Cnamit%40vmware.com%7Cbc4dcc541474462da00b08d6773ab61f%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636827491388051263&amp;sdata=HZNPN4UygwQCqsX8dOajaNeDZyy1O0O4cYeSwu%2BIdO0%3D&amp;reserved=0
> >> 
> >> Hmâ I donât understand what you have in mind (i.e., when would this
> >> assembly directives would be emitted).
> > 
> > For example, it could replace
> > 
> >  callq ____static_call_tramp_my_key
> > 
> > with
> > 
> >  .bundle_align_mode 6
> >  callq ____static_call_tramp_my_key
> >  .bundle_align_mode 0
> > 
> > which ensures the instruction is within a cache line, aligning it with
> > NOPs if necessary.  That would allow my current implementation to
> > upgrade out-of-line calls to inline calls 100% of the time, instead of
> > 95% of the time.
> 
> Heh. I almost wrote based no the feature description that this will add
> unnecessary padding no matter what, but actually (experimentally) it works
> wellâ

Yeah, based on the poorly worded docs, I made the same assumption, until
I tried it.

-- 
Josh