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SubjectRe: [PATCH 0/16 v6] PCI: Linux kernel SR-IOV support
Andi Kleen wrote:
> Anthony Liguori <anthony@codemonkey.ws> writes:
>> What we would rather do in KVM, is have the VFs appear in the host as
>> standard network devices. We would then like to back our existing PV
>> driver to this VF directly bypassing the host networking stack. A key
>> feature here is being able to fill the VF's receive queue with guest
>> memory instead of host kernel memory so that you can get zero-copy
>> receive traffic. This will perform just as well as doing passthrough
>> (at least) and avoid all that ugliness of dealing with SR-IOV in the
>> guest.
>
> But you shift a lot of ugliness into the host network stack again.
> Not sure that is a good trade off.
>
> Also it would always require context switches and I believe one
> of the reasons for the PV/VF model is very low latency IO and having
> heavyweight switches to the host and back would be against that.

I don't think it's established that PV/VF will have less latency than
using virtio-net. virtio-net requires a world switch to send a group of
packets. The cost of this (if it stays in kernel) is only a few
thousand cycles on the most modern processors.

Using VT-d means that for every DMA fetch that misses in the IOTLB, you
potentially have to do four memory fetches to main memory. There will
be additional packet latency using VT-d compared to native, it's just
not known how much at this time.

Regards,

Anthony Liguori


> -Andi
>



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