Re: [Xen-devel] PCI passthrough performance loss with Skylake-SP

["Andreas Kinzler" <hfp@xxxxxxxxx>]

On Tue, 26 Jun 2018 09:47:11 +0200, Paul Durrant <Paul.Durrant@xxxxxxxxxx>  
wrote:
> > > is not affected at all. The test uses standard iperf3 as a client - >  
> > the passed PCI device is not used in the test - so that
> > > just the presence of the passed device will cause the iperf3>  
> > performance to drop from 6.5 gbit/sec (no passthrough)
> > > to 4.5 gbit/sec.
> > I assume that the network interface that you are testing is a PV
> > network interface?
Yes, win-pv.

> > > Any explanation/fixes for that?
> > Are both systems using the same version of Xen and Linux?
Yes, same SSD. Attaching it to different machines.

> I can't necessarily claim credit for the discovery but that is indeed  
> the case, and the sort of performance drop seen is exactly what I'd  
> expect. I recently put a change into the Windows PV drivers to use a  
> ballooned-out region of the guest RAM to host the grant tables instead,  
> which avoids this problem.
> We run with this little hack in XenServer, which also 'fixes' things for  
> guests OS that have not been modified:
> --- a/xen/arch/x86/hvm/mtrr.c
> +++ b/xen/arch/x86/hvm/mtrr.c
I tried the patch and it seems to solve the problem. Thanks.
Is the patch accepted by Xen devs as upstream patch?

Regards Andreas

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