On Thu, Apr 17, 2014 at 03:33:55PM +0300, Michael S. Tsirkin wrote: > On Mon, Mar 31, 2014 at 09:50:44PM +0300, 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. > > > > Unfortunately, this only works if userspace does not need to match on > > access length and data. The implementation adds a separate FAST_MMIO > > bus internally. This serves two purposes: > > - minimize overhead for old userspace that does not use eventfd with lengtth = 0 > > - 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 to handle > > an invalid length) > > > > At the moment, this optimization only has effect for EPT on x86. > > > > It will be possible to speed up MMIO for NPT and MMU using the same > > idea in the future. > > > > With this patch applied, on VMX MMIO EVENTFD is essentially as fast as PIO. > > I was unable to detect any measureable slowdown to non-eventfd MMIO. > > > > Making MMIO faster is important for the upcoming virtio 1.0 which > > includes an MMIO signalling capability. > > > > 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> > > Ping. > Marcelo, any chance you could merge this for 3.16? Applied, note: KVM_CAP_ number change. FYI: "skip_emulated_instruction" breaks instructions which are composed of other parts than just the 0-length eventfd register access, eg. rep mov. -- 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
