On 04.04.2013, at 14:08, Gleb Natapov 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 <m...@redhat.com> >> >> This still uses page fault intercepts which are orders of magnitudes slower >> than hypercalls. 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? >> > It is slower, but not an order of magnitude slower. It become faster > with newer HW. > >> 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. >> > We are trying to avoid PV as much as possible (well this is also PV, > but not guest visible
Also, how is this not guest visible? Who sets KVM_IOEVENTFD_FLAG_PV_MMIO? The comment above its definition indicates that the guest does so, so it is guest visible. +/* + * PV_MMIO - Guest can promise us that all accesses touching this address + * are writes of specified length, starting at the specified address. + * If not - it's a Guest bug. + * Can not be used together with either PIO or DATAMATCH. + */ Alex -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
