Re: Chained VIRTQ_DESC_F_INDIRECT (again!)

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Feb 12, 2015 at 12:14:52PM +1030, Rusty Russell wrote:
> 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.

I agree, explicit is good.
Will you open an issue for that?

> 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.

It does not look very useful, but from a quick look at qemu
code it does seem to support it, so are we sure no one does it?

-- 
MST