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. > 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 ... > 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
