Re: AMD Ryzen KVM/NPT/IOMMU issue

[Alex Williamson <alex.williamson-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Thu, 29 Jun 2017 01:53:57 +0700
Suravee Suthikulpanit <Suravee.Suthikulpanit-5C7GfCeVMHo@public.gmane.org> wrote:

> On 6/29/17 00:26, Steven Walter wrote:
> >> So, I'm trying to reproduce this issue on the Ryzen system w/ the following
> >> setup:
> >>
> >>   * Host kernel v4.11 (with this patch https://lkml.org/lkml/2017/6/23/295)
> >>
> >>   * guest VM RHEL7.3
> >>
> >>   * guest graphic driver = radeon
> >>
> >>   * qemu-system-x86_64 --version
> >>     QEMU emulator version 2.9.50 (v2.9.0-1659-g577caa2-dirty)
> >>
> >>   * kvm-amd npt=1
> >>
> >>   * dGPU is 08:00.0 VGA compatible controller: Advanced Micro Devices, Inc.
> >> [AMD/ATI] Tobago PRO [Radeon R7 360 / R9 360 OEM] (rev 81)
> >>
> >>   * qemu-system-x86_64 -smp 4 -enable-kvm -M q35 -m 4096 -cpu host -bios
> >> /usr/share/qemu/bios.bin -device
> >> ioh3420,bus=pcie.0,addr=1c.0,multifunction=on,port=1,chassis=1,id=root.1
> >> -drive file=/sandbox/vm-images/rhel7.3.qcow2,if=virtio,id=disk0 -net none
> >> -vga none -nodefaults -device
> >> vfio-pci,host=08:00.0,x-vga=on,addr=0.0,multifunction=on,bus=root.1,romfile=/sandbox/vm-images/vbios.rom
> >> -usb -device usb-host,hostbus=3,hostport=1 -device
> >> usb-host,hostbus=3,hostport=3 -device vfio-pci,host=0000:08:00.1 -device
> >> vfio-pci,host=0000:09:00.0
> >>
> >> With this setup, I am able to pass-through the dGPU and run the following
> >> test:
> >>   * Starting up the guest w/ full GNOME GUI on the attached monitor.
> >>   * glxgears (running @ 60 FPS)
> >>   * Playing 1080p HD video on Youtube
> >>
> >> I am not noticing issues here. What kind of test are you running in the
> >> guest VM?  
> > Try running the open source game "torcs" inside the VM.  I think
> > you'll find that there's a very noticeable performance different
> > between npt=1 and npt=0  
> 
> Hm.. actually torcs seems to be running fine w/ npt=1 setup. Although I think my 
> driving skill is ~20% worse :(

A clarification on the issue, it's not that these games/benchmarks
don't work or aren't playable, it's that they run slower (as measured by
framerate) with npt=1 vs npt=0.  A virtualized guest on AMD hardware is
hindered either by lower graphics performance (npt=1) or higher CPU
virtualization overhead (npt=0) for high intensity games or graphics
workloads.  Intel's equivalent ept feature does not have this issue.  I
would encourage looking at the framerate for one mode vs the other
before drawing any conclusions on whether it's working well.  Thanks,

Alex