Re: Xen network domain performance for 10Gb NIC

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Apr 28, 2020 at 04:08:08PM +0000, tosher 1 wrote:
> > Do you get the expected performance from the driver domain when not
> > using it as a backend? Ie: running the iperf benchmarks directly on
> > the driver domain and not on the guest.
> 
> 
> Yes, the bandwidth between the driver domain and the client machine is close 
> to 10Gbits/sec.

Also note that doing grant map / unmap operations from HVM domains is
much more expensive than doing them from PV domains, even without an
IOMMU. Xen needs to modify the domain p2m and issue a flush, and then
the guest (usually) needs to map the newly populated memory in it's
page tables in order to access it.

This and the addition of the IOMMU TLB flush could well explain why
you are seeing such a degraded performance. I'm afraid the only way to
figure out exactly what is causing the bottleneck will be to time the
different operations in Xen. You can compare against a PV dom0 in
order to get an idea.

Roger.