virtio-net: configurable TX queue size

Hi,

 

I want to re-open the discussion left long time ago:

https://lists.gnu.org/archive/html/qemu-devel/2015-11/msg06194.html

, and discuss the possibility of changing the hardcoded (256) TX  queue

size to be configurable between 256 and 1024.

 

The reason to propose this request is that a severe issue of packet drops in

TX direction was observed with the existing hardcoded 256 queue size,

which causes performance issues for packet drop sensitive guest

applications that cannot use indirect descriptor tables. The issue goes away

with 1K queue size. 

 

The concern mentioned in the previous discussion (please check the link

above) is that the number of chained descriptors would exceed 

UIO_MAXIOV (1024) supported by the Linux.

 

From the code,  I think the number of the chained descriptors is limited to

MAX_SKB_FRAGS + 2 (~18), which is much less than UIO_MAXIOV.

Please point out if I missed anything. Thanks.

 

Best,

Wei