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

[Alex Williamson <alex.williamson@redhat.com>]

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.  Thanks,

Alex