[vfio-users] AMD Ryzen Nested Page Table Performance Oddities

[Graham Neville]

I couldn't get a linux VM to boot or install from an ISO with i440fx when
NPT was enabled. I've not tested graphics performance as I'm just using the
linux guest as a plexserver. If I get time I'll try passing the GPU and
testing steam, although I expect the same results as what your seeing.

I've noticed another issue when NPT is enabled with nested virtualisation,
at least when running GNS3. With NPT enabled and GNS3 is running in a linux
guest whenever I try to launch a nested guest the GNS3 guest crashes. With
NPT disabled it works fine.





On Thu, Apr 20, 2017 at 11:49 PM, Nick Sarnie <commendsarnex at gmail.com>
wrote:

> On Thu, Apr 20, 2017 at 12:57 PM, Graham Neville
> <grahamneville at gmail.com> wrote:
> > So turns out a Linux guest with Q35 and OVMF can work with npt on or off.
> > However a Linux guest with i440fx only works with npt disabled.
> >
> > I'll follow the iommu mailing list for updates with regards to NPT
> affecting
> > GPU FPS performance.
> >
> >
> > On 18 Apr 2017 16:14, "Nick Sarnie" <commendsarnex at gmail.com> wrote:
> >>
> >> On Tue, Apr 18, 2017 at 10:09 AM, Graham Neville
> >> <grahamneville at gmail.com> wrote:
> >> > Has there been any feedback on this at all?
> >> >
> >> > I'm still struggling to get a Windows and Linux VM working at the same
> >> > time
> >> > with npt=1. Annoying as I'm having to reboot in to a different kernel
> if
> >> > I
> >> > want to use one or the other.
> >> >
> >> > Are you able to share your Kernel command line and XML for the Linux
> >> > host
> >> > please?
> >> >
> >> >
> >> > On 7 Apr 2017 22:54, "Nick Sarnie" <commendsarnex at gmail.com> wrote:
> >> >
> >> > I can't reproduce that issue, I tested a Fedora VM with both npt 0 and
> >> > 1, of course with the same GPU performance results as windows. I've
> >> > mailed some AMD guys and KVM guys, and hopefully someone can at least
> >> > figure out what is going on. Also, Alex has reproduced this himself on
> >> > both Nvidia and AMD GPUs.
> >> >
> >> > On Fri, Apr 7, 2017 at 5:49 PM, Graham Neville <
> grahamneville at gmail.com>
> >> > wrote:
> >> >> Sarnex,
> >> >>
> >> >> I have similar oddities with npt with Ryzen. As you say with
> >> >> kvm-amd.npt=0
> >> >> the GPU performance is so much better (In my Witcher3 tests I'm going
> >> >> from
> >> >> 40fps to 75fps!). However with npt disabled the Windows10 VM slows
> down
> >> >> an
> >> >> awful lot in general tasks.
> >> >>
> >> >> I've also noticed that I can only run a Linux guest with
> kvm-amd.npt=0,
> >> >> if
> >> >> I
> >> >> have it set to enabled then the Linux guest fails to start. I have
> the
> >> >> same
> >> >> issue even just trying to install Linux from an ISO, it will crash at
> >> >> the
> >> >> GRUB install menu.
> >> >>
> >> >> Hopefully someone knows a fix for this.
> >> >>
> >> >> _______________________________________________
> >> >> vfio-users mailing list
> >> >> vfio-users at redhat.com
> >> >> https://www.redhat.com/mailman/listinfo/vfio-users
> >> >>
> >> >
> >> >
> >>
> >> Alex emailed Paolo, the KVM maintainer, some traces but he couldn't
> >> find anything in them. Please follow the thread on the iommu mailing
> >> list for more details. For now, I think we have to disable NPT.
>
> What do you mean "work"? Do you not see the GPU performance
> degradation with NPT enabled on Q35 and OVMF? All settings work for
> me, it's just that enabling NPT nukes FPS.
>
> Sarnex
>
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