I'm trying out SEV on an EPYC 7282. Everything seems great until I try network operations. I've noticed that KVM guests running with SEV were experiencing significantly reduced network performance. I then added <driver iommu="on"/> to a guest not running with SEV and saw the same problem. I set up a large data transfer from each guest and noted that IOMMU was causing the guest to transfer at a rate 10% of a guest configured without it.
I've tried tinkering with IOMMU-related boot parameters to no avail. I'd simply ignore it and not use IOMMU, except that it is required for virtio devices running with SEV.
Any hints would be welcome. Here is the NIC configuration:
<source dev="ens4f0" mode="bridge"/>
<address type="pci" domain="0x0000" bus="0x01" slot="0x00" function="0x0"/>