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From: Dietmar Maurer <dietmar@proxmox.com>
Date: Tue, 6 Nov 2012 09:01:37 +0000
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Subject: Re: [Qemu-devel] slow virtio network with vhost=on and multiple
 cores
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To: Peter Lieven <lieven-lists@dlhnet.de>, "jan.kiszka@siemens.com" <jan.kiszka@siemens.com>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, "Michael S. Tsirkin" <mst@redhat.com>

OK, bisect point me to this commit:

# git bisect bad
7d37d351dffee60fc7048bbfd8573421f15eb724 is the first bad commit
commit 7d37d351dffee60fc7048bbfd8573421f15eb724
Author: Jan Kiszka <jan.kiszka@siemens.com>
Date:   Thu May 17 10:32:39 2012 -0300

    virtio/vhost: Add support for KVM in-kernel MSI injection
   =20
    Make use of the new vector notifier to track changes of the MSI-X
    configuration of virtio PCI devices. On enabling events, we establish
    the required virtual IRQ to MSI-X message route and link the signaling
    eventfd file descriptor to this vIRQ line. That way, vhost-generated
    interrupts can be directly delivered to an in-kernel MSI-X consumer lik=
e
    the x86 APIC.
   =20
    Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
    Signed-off-by: Avi Kivity <avi@redhat.com>

:040000 040000 1734ddc60cd8a85c7187e93e5a4c02e6d7706cf8 f417e63a684f3b92f5f=
f35d256962a2490890f00 M	hw


This obviously breaks vhost when using multiple cores.


> -----Original Message-----
> From: Peter Lieven [mailto:lieven-lists@dlhnet.de]
> Sent: Dienstag, 06. November 2012 08:47
> To: Dietmar Maurer
> Cc: Stefan Hajnoczi; qemu-devel@nongnu.org; Michael S. Tsirkin
> Subject: Re: [Qemu-devel] slow virtio network with vhost=3Don and multipl=
e
> cores
>=20
> Dietmar Maurer wrote:
> >> > ./x86_64-softmmu/qemu-system-x86_64 -smp sockets=3D1,cores=3D2 -m
> 512 -
> >> hda
> >> > debian-squeeze-netinst.raw -netdev
> >> > type=3Dtap,id=3Dnet0,ifname=3Dtap111i0,vhost=3Don -device
> >> > virtio-net-pci,netdev=3Dnet0,id=3Dnet0
> >> >
> >> > Downloading a larger file with wget inside the guest will show the
> >> problem.
> >> Speed drops from 100MB/s to 15MB/s.
> >> >
> >> > Can someone reproduce that bug? Any ideas how to fix that?
> >>
> >> Which exact QEMU are you using: qemu or qemu-kvm?  Distro package or
> >> from source?  Any patches applied?
> >
> > I just tested with latest sources from
> > git://git.qemu-project.org/qemu.git
> >
> > ./x86_64-softmmu/qemu-system-x86_64 -smp sockets=3D1,cores=3D2 -m 512 -
> hda
> > debian-squeeze-netinst.raw -netdev
> > type=3Dtap,id=3Dnet0,ifname=3Dtap111i0,vhost=3Don -device
> > virtio-net-pci,netdev=3Dnet0,id=3Dnet0 --enable-kvm
> >
> > got incredible bad performance  - can't you reproduce the problem?
>=20
> I have seen a similar problem, ...