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Subject: Chained VIRTQ_DESC_F_INDIRECT (again!)

Hi all,

        I was reading through the spec, and realized that it doesn't
explicitly ban chaining of indirect descriptors.  After virtio-15, it

   An indirect descriptor without next field
   (with flags&VRING_DESC_F_NEXT off) signals the end of the descriptor.

This *implies* that there can't be a "next" after an indirect, but
doesn't explicitly say that a driver shouldn't do that.  That was the
intent, so I suggest adding to Driver Requirements: Indirect

        The driver MUST NOT set the VIRTQ_DESC_F_NEXT flag on a
        descriptor with the VIRTQ_DESC_F_INDIRECT flag set.

Also, the spec allows this combination of direct and indirect descs:

        [direct desc]->[direct desc]->[indirect desc]

Whereas lguest assumes the indirect desc comes first.  Nobody does this,
but we should either specify that the device MUST handle it, or that
the driver MUST NOT do it...


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