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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Andreas Kinzler [mailto:hfp@xxxxxxxxx]
> Sent: 01 July 2018 21:15
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: Roger Pau Monne <roger.pau@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxx
> Subject: Re: [Xen-devel] PCI passthrough performance loss with Skylake-SP
> 
> 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?
> 

Glad the patch solves your problem. I don't think I posted it to xen-devel... 
probably should get round to doing that :-)

  Cheers,

    Paul

> Regards Andreas

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