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SubjectRe: [PATCH RFC] kvm: add PV MMIO EVENTFD
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On 04.04.2013, at 14:19, Gleb Natapov wrote:

> 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.
>>
> Have you bypassed instruction emulation in your testing?

PIO doesn't do instruction emulation.


Alex



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