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Subject: virtio-net: configurable TX queue size

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

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