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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
says:

   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 2.4.5.3.1 Driver Requirements: Indirect
Descriptors:

        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...

Cheers,
Rusty.



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