Messages in this thread | | | From | Nadav Amit <> | Subject | Re: [PATCH v3 0/6] Static calls | Date | Thu, 10 Jan 2019 20:48:31 +0000 |
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> 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&data=02%7C01%7Cnamit%40vmware.com%7Cbc4dcc541474462da00b08d6773ab61f%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636827491388051263&sdata=HZNPN4UygwQCqsX8dOajaNeDZyy1O0O4cYeSwu%2BIdO0%3D&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…
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