On 12.04.2012 11:42, Hans-Kristian Bakke wrote:
> Hi
> 
> For some reason I am not able to get good network performance using
> virtio/vhost-net on Debian KVM host (perhaps also valid for Ubuntu
> hosts then).

The issue has been identified, after Hans-Kristian gave me access
to his machine and I did alot of testing.  And as usual, the root
cause was very stupid... ;)

In last release of debian qemu-kvm package I changed the way how
debian package version string propagates to build procedure -- before
it was a patch grabbing values from debian/version, but in last
release I used --with-pkgversion configure flag.  This resulted in
the following change:

- QEMU emulator version 1.0 (qemu-kvm-1.0 Debian 1.0+dfsg-8), Copyright (c) 
2003-2008 Fabrice Bellard
+ QEMU emulator version 1.0 (Debian qemu-kvm 1.0+dfsg-9), Copyright (c) 
2003-2008 Fabrice Bellard

As it turns out, libvirt parses `qemu -version' output and looks for
" (qemu-kvm-" string in there, and if it is found, libvirt enables
some "extra" features.  Important for us was setting IFF_VNET_HDR
flag for a tap device, -- apparently this flag makes a HUGE difference
in networking speed, especially when using vhost_net.

Obviously this is a change unique to debian, and I never thought
about such an.. "interesting" effect it may give us.

This is a libvirt bug actually, since support of vnet_hdr can be
determined by other means, and since upstream qemu now has almost
everything from qemu-kvm, and it wants to be fast too.  But since
qemu[-kvm] has a long history of changing features, it is difficult
to blame libvirt that much...

Oh well.

Thanks!

/mjt
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