Re: [kvm-devel] [PATCH] Fix e1000 can_receive handler

[Anthony L. <ali...@us...>]

Aurelien Jarno wrote:
> On Wed, May 07, 2008 at 04:40:58PM -0500, Anthony Liguori wrote:
>   
>> The current logic of the can_receive handler is to allow packets whenever the
>> receiver is disabled or when there are descriptors available in the ring.
>>
>> I think the logic ought to be to allow packets whenever the receiver is enabled
>> and there are descriptors available in the ring.
>>     
>
> The current behaviour is actually correct, this is the way QEMU works:
> when the card is stopped, it should always accept packets, and then
> discard them.
>   

The previous patches in my virtio series change that behavior.  Before 
delivering a packet to a VLAN, it checks to see if any of the VLAN 
clients are able to receive a packet.

This is very important for achieving good performance with tap.  With 
virtio-net, we were dropping a ton of packets with tap because there 
weren't descriptors available on the RX ring.

I plan to submit that behavioral change to QEMU upstream along with the 
virtio drivers.  I'm still optimizing phys_page_find() though.  The 
performance impact of switching the ring manipulation to using the 
stl_phys accessors is unacceptable for KVM.

Regards,

Anthony Liguori