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

[Rusty Russell <rusty@xxxxxxxxxxxxxxx>]

On Mon, 2007-03-19 at 18:00 -0800, Zachary Amsden wrote:
> Rusty Russell wrote:
> > *This* was the reason that the current hand-coded calls only clobber %
> > eax.  It was a compromise between native (no clobbers) and others (might
> > need a reg).
> 
> I still don't think this was a good trade.
...
> Xen no longer needs such a register

Hmm, well, if VMI is happy, Xen is happy, and lguest is happy, then
perhaps we're better off with a cc-only clobber rule?  Certainly makes
life simpler.

> > Now, since we decided to allow paravirt_ops operations to be normal C
> > (ie. the patching is optional and done late), we actually push and pop %
> > ecx and %edx.  This makes the call site 10 bytes long, which is a nice
> > size for patching anyway (enough for a movl $0, <addr>, a-la lguest's
> > cli, or movw $0, %gs:<addr> if we supported SMP).
> 
> You can do it in 11 bytes with no clobbers and normal C semantics by 
> linking to a direct address instead of calling to an indirect, but then 
> you need some gross fixup technology in paravirt_patch:
> 
> if (call_addr == (void*)native_sti) {
>   ...
> }

Well, I don't think we need such hacks: since we have to use handcoded
asm and mark the callsites anyway, marking what they're calling is
trivial.

The other idea from "btfixup" is that we can do the patching *much*
earlier, so we don't need the initial code to be valid at all if we
wanted to: we just need room to patch in a call insn.  We could then
generate trampolines which do the necessary pushes & pops automatically
for backends which want to use C calling conventions.

Perhaps it's time for code and benchmarks?

Rusty.


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