Re: [virtio-comment] virtio-net : 5.1.6.2 descriptor-chains on transmitq

[Stefan Hajnoczi <stefanha@redhat.com>]

On Wed, Mar 09, 2022 at 06:00:08PM +0500, Awais Masood wrote:
> Hi,
> 
> Seeking some clarity on:
> 
>  5.1.6.2
>  Packet Transmission
> 
>  5. The header and packet are added as one output descriptor to the
> transmitq, and the device is notified of the new entry
> 
> Does it imply no descriptor-chains for packets in transmitq? If
> descriptor-chains are allowed on transmitq then why does it not state
> 'output buffer' instead of 'output descriptor'?

Good point, the spec is misleading.

TX buffers can be chains and/or use indirect descriptors. I think it
should read:

  The header and packet output descriptors are added as one buffer to
  the transmitq, and the device is notified of the new entry

There are no restrictions on descriptor framing, it would violate the
spec:

  2.6.4 Message Framing

  The framing of messages with descriptors is independent of the
  contents of the buffers. For example, a network transmit buffer
  consists of a 12 byte header followed by the network packet. This
  could be most simply placed in the descriptor table as a 12 byte
  output descriptor followed by a 1514 byte output descriptor, but it
  could also consist of a single 1526 byte output descriptor in the case
  where the header and packet are adjacent, or even three or more
  descriptors (possibly with loss of efficiency in that case). 

Would you like to send a patch to update the spec? The guidelines for
contributing spec changes are here:
https://github.com/oasis-tcs/virtio-spec/blob/master/CONTRIBUTING.md

Stefan

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