Re: [RFC PATCH 0/3] generic hypercall support

Marcelo Tosatti
2009-05-07T23:36:56Z

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.

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. 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)?