On Wed, Jun 28, 2017 at 4:33 PM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > > On 28/06/2017 21:52, Graham Neville wrote: >> 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. > > Nested AMD needs some care. It's known, but time has been lacking... > > Paolo > _______________________________________________ > iommu mailing list > iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx > https://lists.linuxfoundation.org/mailman/listinfo/iommu Hi Suravee, Thanks a lot for helping. Torcs does not appear graphically demanding on modern hardware, so this issue may not be easily noticeable. I was able to easily reproduce the problem using the Unigine Heaven benchmark, but I'm sure anything moderately graphically demanding would show a performance loss with NPT enabled. As an example, when I tested this with Fedora on my RX480, I got around 30-35 FPS with NPT on and around 55-60 with NPT off. Let me know if you need any more information or have any questions. (no problem John, thanks a lot for taking interest in this) Thanks again, Sarnex
