[vfio-users] AMD Ryzen KVM/NPT/IOMMU issue

[Graham Neville]

Although not related to graphics card performance, there is definitely
another issue with regards to running KVM nested L2 guests when npt=1.

Thought I'd mention this in case it helps with identifying performance
issues with NPT.

I'm unable to start any L2 guests with KVM acceleration (--enable-kvm). As
soon as it attempts to bring up the L2 guest the L1 host crashes, L0 host
remains online. Nothing is printed in either L1 or L0's dmesg.

My L0 is running Arch with 4.11.0-rc6, with qemu 2.8.0. I've tried
different L1 hosts (Ubuntu,Arch) and different kernels right to 4.12-rc5
kernel, along with different qemu versions.

This used to work fine with my Intel i7-4770s setup.

With npt=0, L2 guests can start but performance is dier.
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