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    SubjectRe: [RFC PATCH 0/3] generic hypercall support
    On Thu, May 07, 2009 at 01:03:45PM -0400, Gregory Haskins wrote:
    > Chris Wright wrote:
    > > * Gregory Haskins ( wrote:
    > >
    > >> Chris Wright wrote:
    > >>
    > >>> VF drivers can also have this issue (and typically use mmio).
    > >>> I at least have a better idea what your proposal is, thanks for
    > >>> explanation. Are you able to demonstrate concrete benefit with it yet
    > >>> (improved latency numbers for example)?
    > >>>
    > >> I had a test-harness/numbers for this kind of thing, but its a bit
    > >> crufty since its from ~1.5 years ago. I will dig it up, update it, and
    > >> generate/post new numbers.
    > >>
    > >
    > > That would be useful, because I keep coming back to pio and shared
    > > page(s) when think of why not to do this. Seems I'm not alone in that.
    > >
    > > thanks,
    > > -chris
    > >
    > I completed the resurrection of the test and wrote up a little wiki on
    > the subject, which you can find here:
    > Hopefully this answers Chris' "show me the numbers" and Anthony's "Why
    > reinvent the wheel?" questions.
    > I will include this information when I publish the updated v2 series
    > with the s/hypercall/dynhc changes.
    > Let me know if you have any questions.


    I think comparison is not entirely fair. You're using
    KVM_HC_VAPIC_POLL_IRQ ("null" hypercall) and the compiler optimizes that
    (on Intel) to only one register read:

    nr = kvm_register_read(vcpu, VCPU_REGS_RAX);

    Whereas in a real hypercall for (say) PIO you would need the address,
    size, direction and data.

    Also for PIO/MMIO you're adding this unoptimized lookup to the

    pio_dev = vcpu_find_pio_dev(vcpu, port, size, !in);
    if (pio_dev) {
    kernel_pio(pio_dev, vcpu, vcpu->arch.pio_data);
    return 1;

    Whereas for hypercall measurement you don't. I believe a fair comparison
    would be have a shared guest/host memory area where you store guest/host
    TSC values and then do, on guest:

    ... back to host

    And then calculate the difference (minus guests TSC_OFFSET of course)?

     \ /
      Last update: 2009-05-08 01:39    [W:0.027 / U:217.460 seconds]
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