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

[Alexander Graf]

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@xxxxxxxxxx>
>> 
>> 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.

> 
>> 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

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