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SubjectRe: [PATCH 2/2] vmx: speed up wildcard MMIO EVENTFD
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@redhat.com>
>
> 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.



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