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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jan 20, 2011 at 06:35:46PM -0700, Alex Williamson wrote:
> On Thu, 2011-01-20 at 18:23 -0600, Anthony Liguori wrote:
> > On 01/20/2011 10:07 AM, Michael S. Tsirkin wrote:
> > > 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<mst@redhat.com>
> > >>>        
> > >> 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.
> > >    
> > 
> > In the very least, there needs to be a vhost=force.
> > 
> > Having some sort of friendly default policy is fine but we need to 
> > provide a mechanism for a user to have the final say.  If you want to 
> > redefine vhost=on to really mean, use the friendly default, that's fine 
> > by me, but only if the vhost=force option exists.
> > 
> > I actually would think libvirt would want to use vhost=force.  Debugging 
> > with vhost=on is going to be a royal pain in the ass if a user reports 
> > bad performance.  Given the libvirt XML, you can't actually tell from 
> > the guest and the XML whether or not vhost was actually in use or not.
> 
> If we add a force option, let's please distinguish hotplug from VM
> creation time.  The latter can abort.  Hotplug should print an error and
> fail the initfn.

It can't abort at init - MSI is disabled at init, it needs to be enabled
by the guest later. And aborting the guest in the middle of the run
is a very bad idea.

What vhostforce=true will do is force vhost backend to be used even if
it is slower.

>  Thanks,
> 
> Alex