HVM performance once again

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

Hi,

I returned to HVM performance once again, this time looking at PCI
passthrough impact evaluating network throughput.
The setup:
 - Xen 4.17
 - Linux 6.3.2 in all domU
 - iperf -c running in a PVH (call it "client")
 - iperf -s running in a HVM (call it "server")
 - client's netfront has a backend directly in server
 - frontend's "trusted" is set to 0
 - HVM have qemu in a stubdomain in all the cases
 - no intentional differences about HVM besides presence of a PCI device
   (it is a network card, but it was not involved in the traffic)

And now the results:
 - server is plain HVM: ~6Gbps
 - server is HVM and has some PCI passthrough: ~3Gbps

Any idea why such huge difference?

One difference I see when comparing the logs, is 64MB swiotlb initalized
in no-PCI case, but I'm not sure if that's really relevant...

Both dmesg attached.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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