Re: AMD Ryzen KVM/NPT/IOMMU issue

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 3 May 2017 12:28:35 -0400
Nick Sarnie <commendsarnex@gmail.com> wrote:

> On Wed, May 3, 2017 at 10:37 AM, Matthias Ehrenfeuchter <efeu@markju.com> wrote:
> > Hi,
> >
> > There are a lot of messages/threads out there about bad performance while
> > using AMDs Ryzen with KVM GPU passthrough. It revolves all on
> > enabling/disabling npt, while enabled overall VM performance is nice but the
> > GPU performance gives me about 20% (and a lot of drops to zero GPU usage,
> > while CPU/Disk/Ram also doing nothing) compared to npt disabled. But while
> > npt is disabled overall VM performance is like beeing on 4x86 with floppy
> > disk as only storage. (Ex. it takes 2 seconds just to open startmenu while
> > host and vm are in idle, and neither CPU pinning, changing CPU model,
> > changing storage device nor using hugepages changed anything).
> >
> > So everything I read pointed to a bug in the npt implementation? Anything I
> > could do to get closer to the "thing" issuing this?
> >
> > Best Regards
> >
> > efeu
> > _______________________________________________
> > iommu mailing list
> > iommu@lists.linux-foundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/iommu  
> 
> I heard from Joerg that it might be related to a lower intercept rate
> being used when NPT is enabled, but we haven't been able to find a way
> to trace that to confirm.

Joerg/Paolo, any ideas how we might debug this?  Anyone from AMD
watching?  Thanks,

Alex