Re: [Qemu-devel] Ideas wiki for GSoC 2010

Joerg Roedel
2010-03-15T13:24:50Z

On Mon, Mar 15, 2010 at 03:11:42PM +0200, Avi Kivity wrote:
> On 03/15/2010 03:03 PM, Joerg Roedel wrote:
>>
>>>> I will add another project - iommu emulation. Could be very useful
>>>> for doing device assignment to nested guests, which could make
>>>> testing a lot easier.
>>>>
>>> Our experiments show that nested device assignment is pretty much
>>> required for I/O performance in nested scenarios.
>>>
>> Really? I did a small test with virtio-blk in a nested guest (disk read
>> with dd, so not a real benchmark) and got a reasonable read-performance
>> of around 25MB/s from the disk in the l2-guest.
>>
>>
>
> Your guest wasn't doing a zillion VMREADs and VMWRITEs every exit.
>
> I plan to reduce VMREAD/VMWRITE overhead for kvm, but not much we can do
> for other guests.

Does it matter for the ept-on-ept case? The initial patchset of
nested-vmx implemented it and they reported a performance drop of around
12% between levels which is reasonable. So I expected the loss of
io-performance for l2 also reasonable in this case. My small measurement
was also done using npt-on-npt.

Joerg