Re: [PATCH RFC] kvm: add PV MMIO EVENTFD

["Michael S. Tsirkin" <mst@redhat.com>]

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@redhat.com>
> 
> 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