[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable
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. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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