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

[Anthony Liguori <anthony@codemonkey.ws>]

On 01/21/2011 03:55 AM, Michael S. Tsirkin wrote:
> 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.
>    

vhost=on,vhostforce=false              use vhost if we think it will 
improve performance
vhost=on,vhostforce=true               always use vhost
vhost=off,vhostforce=*                    do not use vhost

Regards,

Anthony Liguori

>    
>>   Thanks,
>>
>> Alex
>>      
>