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

[Graham Neville]

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.
>
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