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



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 gmail com> wrote:
On Tue, Apr 18, 2017 at 10:09 AM, Graham Neville
<grahamneville 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 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 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 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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