On 04.04.2013, at 13:04, Michael S. Tsirkin wrote: > On Thu, Apr 04, 2013 at 01:57:34PM +0200, Alexander Graf wrote: >> >> On 04.04.2013, at 12:50, Michael S. Tsirkin wrote: >> >>> With KVM, MMIO is much slower than PIO, due to the need to >>> do page walk and emulation. But with EPT, it does not have to be: we >>> know the address from the VMCS so if the address is unique, we can look >>> up the eventfd directly, bypassing emulation. >>> >>> Add an interface for userspace to specify this per-address, we can >>> use this e.g. for virtio. >>> >>> The implementation adds a separate bus internally. This serves two >>> purposes: >>> - minimize overhead for old userspace that does not use PV MMIO >>> - minimize disruption in other code (since we don't know the length, >>> devices on the MMIO bus only get a valid address in write, this >>> way we don't need to touch all devices to teach them handle >>> an dinvalid length) >>> >>> At the moment, this optimization is only supported for EPT on x86 and >>> silently ignored for NPT and MMU, so everything works correctly but >>> slowly. >>> >>> TODO: NPT, MMU and non x86 architectures. >>> >>> The idea was suggested by Peter Anvin. Lots of thanks to Gleb for >>> pre-review and suggestions. >>> >>> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx> >> >> This still uses page fault intercepts which are orders of magnitudes >> slower than hypercalls. > > Not really. Here's a test: > compare vmcall to portio: > > vmcall 1519 > ... > outl_to_kernel 1745 > > compare portio to mmio: > > mmio-wildcard-eventfd:pci-mem 3529 > mmio-pv-eventfd:pci-mem 1878 > portio-wildcard-eventfd:pci-io 1846 > > So not orders of magnitude. https://dl.dropbox.com/u/8976842/KVM%20Forum%202012/MMIO%20Tuning.pdf Check out page 41. Higher is better (number is number of loop cycles in a second). My test system was an AMD Istanbul based box. > >> Why don't you just create a PV MMIO hypercall >> that the guest can use to invoke MMIO accesses towards the host based >> on physical addresses with explicit length encodings? >> That way you simplify and speed up all code paths, exceeding the speed >> of PIO exits even. It should also be quite easily portable, as all >> other platforms have hypercalls available as well. >> >> >> Alex > > I sent such a patch, but maintainers seem reluctant to add hypercalls. > Gleb, could you comment please? > > A fast way to do MMIO is probably useful in any case ... Yes, but at least according to my numbers optimizing anything that is not hcalls is a waste of time. Alex -- 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
