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

From: Anthony Liguori <anthony@codemonkey.ws>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: kvm@vger.kernel.org, Juan Quintela <quintela@redhat.com>,
	Jes.Sorensen@redhat.com, jasowang@redhat.com,
	qemu-devel@nongnu.org,
	Alex Williamson <alex.williamson@redhat.com>
Subject: Re: [Qemu-devel] [PATCH] vhost: force vhost off for non-MSI guests
Date: Thu, 20 Jan 2011 18:23:36 -0600	[thread overview]
Message-ID: <4D38D208.4090403@codemonkey.ws> (raw)
In-Reply-To: <20110120160718.GA24652@redhat.com>

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.

Regards,

Anthony Liguori

> 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"
>
>
>    