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

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

On Fri, Feb 26, 2010 at 09:23:01AM -0600, Anthony Liguori wrote:
> 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<mst@redhat.com>
>>>>
>>>>        
>>> 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

Yes, default to on with 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.

Speaking of vlan filtering etc?  It's just a matter of time before it
supports all interesting features. Kernel support is there in net-next
already, userspace should be easy too. I should be able to code it up
once I finish bothering about upstream merge (hint hint :)).

> 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.

It's hard for me to see how it reduces VM security. If it does, it's
not by design and will be fixed.

> 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.

Not yet, but we'll get there.

>> 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