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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