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

[Anthony Liguori <anthony@codemonkey.ws>]

On 01/21/2011 03:48 AM, Michael S. Tsirkin wrote:
> On Thu, Jan 20, 2011 at 06:23:36PM -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.
>>      
> OK, I will add that, probably as a separate flag as vhost
> is a boolean.  This will get worse performance but it will be what the
> user asked for.
>
>    
>> 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.
>>      
> Yes you can: check MSI enabled in the guest, if it is -
> check vhost enabled in the XML. Not that bad at all, is it?
>    

Until you automatically detect level triggered interrupt support for 
irqfd.  This means it's also dependent on a kernel feature too.

Is there any way to tell in QEMU that vhost was silently disabled?

Regards,

Anthony Liguori

>    
>> Regards,
>>
>> Anthony Liguori
>>      
> We get worse performance without MSI anyway, how is this different?
>
>    
>>> 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"
>>>
>>>
>>>        
>