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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jun 25, 2018 at 05:32:58PM +0200, Andreas Kinzler wrote:
> I am currently testing PCI passthrough on the Skylake-SP platform using a
> Supermicro X11SPi-TF mainboard. Using PCI passthrough (an LSI SAS HBA)
> causes severe performance loss on the Skylake-SP platform while Xeon E3 v5
> 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?

> Any explanation/fixes for that?

Are both systems using the same version of Xen and Linux?

Paul discovered that (recent?) Linux versions the grant table is
allocated in a MMIO area, which has the memory cache attribute set to
uncached. This has a noticeable impact on performance when the guest
also has a passthrough device.

Note that this is not an issue without passthrough because Xen forces
the cache attributes of memory to WB in that case, regardless of the
guest choice.

Roger.

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