On 04.04.2013, at 14:19, Gleb Natapov wrote: > On Thu, Apr 04, 2013 at 02:09:53PM +0200, Alexander Graf wrote: >> >> 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. >> > Have you bypassed instruction emulation in your testing? PIO doesn't do instruction emulation. 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
