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