[Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options

[Anthony Liguori]

On 02/26/2010 08:51 AM, Michael S. Tsirkin wrote:
> On Thu, Feb 25, 2010 at 01:47:27PM -0600, Anthony Liguori wrote:
>    
> > On 02/25/2010 12:28 PM, Michael S. Tsirkin wrote:
> >      
> > > This adds vhost binary option to tap, to enable vhost net accelerator.
> > > Default is off for now, we'll be able to make default on long term
> > > when we know it's stable.
> > >
> > > vhostfd option can be used by management, to pass in the fd. Assigning
> > > vhostfd implies vhost=on.
> > >
> > > Signed-off-by: Michael S. Tsirkin<address@hidden>
> > >
> > >        
> > Since the thinking these days is that macvtap and tap is pretty much all
> > we'll ever need for vhost-net, perhaps we should revisit -net vhost vs.
> > -net tap,vhost=X?
> >
> > I think -net vhost,fd=X makes a lot more sense than -net
> > tap,vhost=on,vhostfd=X.
> >
> > Regards,
> >
> > Anthony Liguori
> >      
> We'll have to duplicate all tap options.
> I think long term we will just make vhost=on the default.
>    

I don't think we can.  vhost only works when using KVM and it doesn't 
support all of the features of userspace virtio.  Since it's in upstream 
Linux without supporting all of the virtio-net features, it's something 
we're going to have to deal with for a long time.

Furthermore, vhost reduces a virtual machine's security.  It offers an 
impressive performance boost (particularly when dealing with 10gbit+ 
networking) but for a user that doesn't have such strong networking 
performance requirements, I think it's reasonable for them to not want 
to make a security trade off.

One reason I like -net vhost is that it's a much less obscure syntax and 
it's the sort of thing that is easy to tell users that they should use.  
I understand you're argument for -net tap if you assume vhost=on will 
become the default because that means that users never really have to be 
aware of vhost once it becomes the default.  But as I said above, I 
don't think it's reasonable to make it on by default with -net tap.

> Users do not really care about vhost, it just makes tap
> go fater. So promoting it to 1st class options is wrong IMO.
>    

User's should care about vhost because it impacts the features supported 
by the virtual machine and it has security ramifications.  It's a great 
feature and I think the most users will want to use it, but I do think 
it's something that users ought to be aware of.

Regards,

Anthony Liguori