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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?

Regards Andreas

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