Re: [Qemu-devel] Using virtio-net and vhost_net on an ARM machine using qemu-system-arm & KVM

[Barak Wasserstrom]

Peter,

Thanks - I got virtio-net-device running now, but performance is terrible.

When i look at the guest's ethernet interface features (ethtool -k eth0) i see all offload features are disabled.

I'm using a virtual tap on the host (tap0 bridged to eth3).

On the tap i also see all offload features are disabled, while on br0 and eth3 i see the expected offload features.

Can this explain the terrible performance i'm facing?

If so, how can this be changed?

If not, what else can cause such bad performance?

Do you know if vhost_net can be used on ARM Cortex A15 host/guest, even though the guest doesn't support PCI & MSIX?

Regards,

Barak

On Sun, Jan 12, 2014 at 11:15 PM, Peter Maydell <address@hidden> wrote:

On 9 January 2014 12:25, Barak Wasserstrom <address@hidden> wrote:
> Hi,
> I would like to utilize virtio-net and vhost_net on an ARM Cortex A15
> machine using qemu-system-arm & KVM.
> I have few questions:
> 1. Do i need to build qemu-system-arm myself, or apt-get install it? When i
> apt-get install it i get "KVM not supported for this target. "kvm"
> accelerator does not exist. No accelerator found!".

This sounds like either:
 (1) you're using too old a version of QEMU and need a newer one
 (2) you configured QEMU without KVM support

Provided you have QEMU 1.6 or later it shouldn't matter whose
version you're using.

> 2. Do i need to execute qemu-system-arm directly or through virsh? Does it
> matter?

I know nothing about virsh but I don't expect it matters. It's
probably easier to get things working by running qemu-system-arm
directly first, before you try to work out how to get virsh to start
qemu with the correct arguments.

> 3. Must i use a machine that supports PCI controller or not? And if so,
> which machine supports it? I saw that 'virt' and 'vexpress' don't support
> it.

No. For KVM to work you need to use an A15 guest CPU; there
are no A15 boards in QEMU which have a PCI controller. So
instead you have to use the vexpress-a15 or virt machine's
virtio-mmio support. Note that generally the command line syntax
for this is different from that used by x86: you need to create
virtio-*-device devices, not virtio-* or virtio-*-pci devices, and you
can't rely on shorthands like if=virtio. So for instance for a block
device you need
  -drive if=none,file=root,id=foo -device virtio-blk-device,drive=foo

thanks
-- PMM