virtio-spec: Clarify /wrt maintaining a split-queue descriptors

Hi all,

I'm wondering if the spec is clear enough regarding who is in charge of
setting fields in a descriptor of split virtqueues. Background: I just
made the mistake in an experimental backend of writing back the number
of bytes pushed into a buffer not only in virtq_used_elem but also the
referenced virtq_desc - with unpleasant effects for the guest.

The spec in section 2.6.8 reads to me like virtq_used_elem.len is rather
informative. At least this section leaves it fuzzy to me whether there
are other sources from where a driver can find out how much a device has
delivered - and misled me to also update virtq_desc.len in the device.

But maybe I'm missing a clear note in another section.

Jan

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Siemens AG, Corporate Technology, CT RDA IOT SES-DE
Corporate Competence Center Embedded Linux