Re: [PATCH 0/16 v6] PCI: Linux kernel SR-IOV support

[Zhao, Yu]

Avi Kivity wrote:
> Anthony Liguori wrote:
> > 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.
>
> If the IOTLB has intermediate TLB entries like the processor, we're 
> talking just one or two fetches.  That's a lot less than the cacheline 
> bouncing that virtio and kvm interrupt injection incur right now.

The PCI SIG Address Translation Service (ATS) specifies a way that uses 
an Address Translation Cache (ATC) in the Endpoint to reduce the latency.

The Linux kernel support for ATS capability will come soon.

Thanks,
Yu
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