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    SubjectRe: [RFC PATCH 0/3] generic hypercall support
    Marcelo Tosatti wrote:
    > On Thu, May 07, 2009 at 01:03:45PM -0400, Gregory Haskins wrote:
    >
    >> Chris Wright wrote:
    >>
    >>> * Gregory Haskins (ghaskins@novell.com) 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:
    >>
    >> http://developer.novell.com/wiki/index.php/WhyHypercalls
    >>
    >> 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.
    >>
    >
    > Greg,
    >
    > 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.
    >

    Hi Marcelo,
    I'll have to respectfully disagree with you here. What you are
    proposing is actually a different test: a 4th type I would call "PIO
    over HC". It is distinctly different than the existing MMIO, PIO, and
    HC tests already present.

    I assert that the current HC test remains valid because for pure
    hypercalls, the "nr" *is* the address. It identifies the function to be
    executed (e.g. VAPIC_POLL_IRQ = null), just like the PIO address of my
    nullio device identifies the function to be executed (i.e.
    nullio_write() = null)

    My argument is that the HC test emulates the "dynhc()" concept I have
    been talking about, whereas the PIOoHC is more like the
    pv_io_ops->iowrite approach.

    That said, your 4th test type would actually be a very interesting
    data-point to add to the suite (especially since we are still kicking
    around the notion of doing something like this). I will update the
    patches.


    > Also for PIO/MMIO you're adding this unoptimized lookup to the
    > measurement:
    >
    > pio_dev = vcpu_find_pio_dev(vcpu, port, size, !in);
    > if (pio_dev) {
    > kernel_pio(pio_dev, vcpu, vcpu->arch.pio_data);
    > complete_pio(vcpu);
    > return 1;
    > }
    >
    > Whereas for hypercall measurement you don't.

    In theory they should both share about the same algorithmic complexity
    in the decode-stage, but due to the possible optimization you mention
    you may have a point. I need to take some steps to ensure the HC path
    isn't artificially simplified by GCC (like making the execute stage do
    some trivial work like you mention below).

    > 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:
    >
    > rdtscll(&shared_area->guest_tsc);
    > pio/mmio/hypercall
    > ... back to host
    > rdtscll(&shared_area->host_tsc);
    >
    > And then calculate the difference (minus guests TSC_OFFSET of course)?
    >
    >
    I'm not sure I need that much complexity. I can probably just change
    the test harness to generate an ioread32(), and have the functions
    return the TSC value as a return parameter for all test types. The
    important thing is that we pick something extremely cheap (yet dynamic)
    to compute so the execution time doesn't invalidate the measurement
    granularity with a large constant.

    Regards,
    -Greg


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