[Xen-devel] Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable

[Zachary Amsden <zach@xxxxxxxxxx>]

Jeremy Fitzhardinge wrote:
> I think the suggestion is much simpler.  If you convince gcc/binutils to
> leave the .reloc section in vmlinux, and make that available to the
> kernel itself, then you can scan all the kernel's relocs to find ones
> which refer to paravirt_ops, and use those to determine which are
> callsites that can be patched.
>   
Yes, that is pretty nice.

> The main upside is that all the callsites are just normal C calls;
> there's no special syntax or strange macros, and we get the full benefit
> of typechecking, etc.
>
> But I can see a few downsides compared the current scheme:
>
>    1. Identifying the callsites is a somewhat hackish process of looking
>       at a reloc and doing a bit of dissassembly to see what is using
>       the reloc, to identify calls and jumps
>    2. There's nothing explicit to tell us how much space there is to
>       patch into; we just have to assume sizeof(indirect call/jmp)
>    3. There's no information about the register environment at the
>       callsite, so we just have to adopt normal C ABI rules.  For the
>       patch sites in hand-written asm, this could be tricky.
>    4. gcc could do strange things which prevent detection of patch
>       sites.  For example, it might CSE the value of, say,
>       paravirt_ops.irq_enable, which would be a reasonable optimisation,
>       but prevent any of the resulting indirect calls from being
>       patched.  In general it relies on gcc to generate identifiable
>       callsites, which is a bit unpredictable.
>    5. There's still a moderate amount of binutils hackery to get the
>       relocs into the right form, and there's plenty of scope for it to
>       screw up.
>   
And yes, those are nasty points.  I think I'd be interested in seeing 
more discussion on it, perhaps those issues could be worked out.
> > [ Roswell technology deleted ]
> >     
Nack.  Everyone needs Roswell technology.

Actually, that was not a serious proposal.  I think the effort and 
complexity would likely not justify the gain.  But I still had to throw 
it out, since it is what we use on Betelgeuse.
Z

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