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

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

On Thu, Apr 04, 2013 at 02:09:53PM +0200, Alexander Graf wrote:
> 
> On 04.04.2013, at 13:04, Michael S. Tsirkin 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 <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.
> 
> https://dl.dropbox.com/u/8976842/KVM%20Forum%202012/MMIO%20Tuning.pdf
> 
> Check out page 41. Higher is better (number is number of loop cycles in a second). My test system was an AMD Istanbul based box.

Wow 2x difference.  The difference seems to be much smaller now. Newer
hardware? Newer software?

> > 
> >> 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 ...
> 
> Yes, but at least according to my numbers optimizing anything that is not hcalls is a waste of time.
> 
> 
> Alex

This was the implementation: 'kvm_para: add mmio word store hypercall'.
Again, I don't mind.

-- 
MST