On Thu, May 07, 2009 at 01:03:45PM -0400, Gregory Haskins wrote: > Chris Wright wrote: > > * Gregory Haskins (ghaskins@xxxxxxxxxx) 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)? -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
