Subject: Re: [net-next RFC PATCH 5/5] virtio-net: flow director support

From: Sridhar Samudrala <[email-masked]>
Date: Tue, 06 Dec 2011 07:42:54 -0800

On 12/6/2011 5:15 AM, Stefan Hajnoczi wrote:
> On Tue, Dec 6, 2011 at 10:21 AM, Jason Wang<[email-masked]>  wrote:
>> On 12/06/2011 05:18 PM, Stefan Hajnoczi wrote:
>>> On Tue, Dec 6, 2011 at 6:33 AM, Jason Wang<[email-masked]>    wrote:
>>>> On 12/05/2011 06:55 PM, Stefan Hajnoczi wrote:
>>>>> On Mon, Dec 5, 2011 at 8:59 AM, Jason Wang<[email-masked]>
>>>>>   wrote:
>>> The vcpus are just threads and may not be bound to physical CPUs, so
>>> what is the big picture here?  Is the guest even in the position to
>>> set the best queue mappings today?
>>
>> Not sure it could publish the best mapping but the idea is to make sure the
>> packets of a flow were handled by the same guest vcpu and may be the same
>> vhost thread in order to eliminate the packet reordering and lock
>> contention. But this assumption does not take the bouncing of vhost or vcpu
>> threads which would also affect the result.
> Okay, this is why I'd like to know what the big picture here is.  What
> solution are you proposing?  How are we going to have everything from
> guest application, guest kernel, host threads, and host NIC driver
> play along so we get the right steering up the entire stack.  I think
> there needs to be an answer to that before changing virtio-net to add
> any steering mechanism.
>
>
Yes. Also the current model of  a vhost thread per VM's interface 
doesn't help with packet steering
all the way from the guest to the host physical NIC.

I think we need to have vhost thread(s) per-CPU that can handle packets 
to/from physical NIC's
TX/RX queues. Currently we have a single vhost thread for a VM's i/f 
that handles all the packets from
various flows coming from a multi-queue physical NIC.

Thanks
Sridhar