Re: [Qemu-devel] [PATCH] vhost: force vhost off for non-MSI guests

[Michael S. Tsirkin]

On Thu, Jan 20, 2011 at 09:43:57AM -0600, Anthony Liguori wrote:
> On 01/20/2011 09:35 AM, Michael S. Tsirkin wrote:
> >When MSI is off, each interrupt needs to be bounced through the io
> >thread when it's set/cleared, so vhost-net causes more context switches and
> >higher CPU utilization than userspace virtio which handles networking in
> >the same thread.
> >
> >We'll need to fix this by adding level irq support in kvm irqfd,
> >for now disable vhost-net in these configurations.
> >
> >Signed-off-by: Michael S. Tsirkin<address@hidden>
> 
> I actually think this should be a terminal error.  The user asks for
> vhost-net, if we cannot enable it, we should exit.
> 
> Or we should warn the user that they should expect bad performance.
> Silently doing something that the user has explicitly asked us not
> to do is not a good behavior.
> 
> Regards,
> 
> Anthony Liguori

The issue is that user has no control of the guest, and can not know
whether the guest enables MSI. So what you ask for will just make
some guests fail, and others fail sometimes.
The user also has no way to know that version X of kvm does not expose a
way to inject level interrupts with irqfd.

We could have *another* flag that says "use vhost where it helps" but
then I think this is what everyone wants to do, anyway, and libvirt
already sets vhost=on so I prefer redefining the meaning of an existing
flag.

Maybe this is best handled by a documentation update?

We always said:
            "                use vhost=on to enable experimental in kernel 
accelerator\n"

note 'enable' not 'require'. This is similar to how we specify
nvectors : you can not make guest use the feature.

How about this:

diff --git a/qemu-options.hx b/qemu-options.hx
index 898561d..3c937c1 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -1061,6 +1061,7 @@ DEF("net", HAS_ARG, QEMU_OPTION_net,
     "                use vnet_hdr=off to avoid enabling the IFF_VNET_HDR tap 
flag\n"
     "                use vnet_hdr=on to make the lack of IFF_VNET_HDR support 
an error condition\n"
     "                use vhost=on to enable experimental in kernel 
accelerator\n"
+    "                (note: vhost=on has no effect unless guest uses MSI-X)\n"
     "                use 'vhostfd=h' to connect to an already opened vhost net 
device\n"
 #endif
     "-net 
socket[,vlan=n][,name=str][,fd=h][,listen=[host]:port][,connect=host:port]\n"