Re: [PATCH v3 0/6] Static calls

[Nadav Amit <namit@vmware.com>]

> On Jan 10, 2019, at 12:32 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> 
> On Thu, Jan 10, 2019 at 07:45:26PM +0000, Nadav Amit wrote:
>>>> I’m not GCC expert either and writing this code was not making me full of
>>>> joy, etc.. I’ll be happy that my code would be reviewed, but it does work. I
>>>> don’t 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, I’ll 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@vmware.com/ ?

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.

>>>>> 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 don’t 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
>> 
>> Hm… I don’t 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
well…