[virtio-comment] Re: [PATCH v3 4/4] Add CCW configuration field "indirect_num"

[Cornelia Huck <cohuck@redhat.com>]

On Wed, Mar 16 2022, Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> This new CCW configuration field allows to negotiate a more fine
> graded maximum lenght of indirect descriptor chains.
>
> Bump CCW virtio revision to 3 and make the existence of this new
> field "indirect_num" dependant on revision 3.
>
> Fixes: https://github.com/oasis-tcs/virtio-spec/issues/122
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> ---
>  content.tex | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
>

(...)

> @@ -2581,12 +2583,17 @@ \subsubsection{Configuring a Virtqueue}\label{sec:Virtio Transport Options / Vir
>  struct vq_config_block {
>          be16 index;
>          be16 max_num;
> +        be32 indirect_num; /* since virtio-ccw rev. 3 */

Maybe make it max_indirect_num (to mirror max_num?)

>  };
>  \end{lstlisting}
>  
>  The requested number of buffers for queue \field{index} is returned in
>  \field{max_num}.
>  
> +Since revision 3, \field{indirect_num} exists, which is supposed to reflect the
> +Queue Indirect Size (i.e. the maximum length of indirect descriptor tables)
> +supported by device for this queue.

It still depends on the feature flag, though.

Maybe

"If revision 2 or lower is set, struct vq_config_block extends up to
\field{max_num}.

If revision 3 or higher is set, struct vq_config_block extends to
\field{max_indirect_num}. If VIRTIO_RING_F_INDIRECT_SIZE is negotiated,
\field{indirect_max_num} contains the maximum Queue Indirect Size
(i.e. the maximum length of indirect descriptor tables) supported by the
device for this queue; otherwise, its contents are unpredictable."

> +
>  Afterwards, CCW_CMD_SET_VQ is issued by the driver to inform the
>  device about the location used for its queue. The transmitted
>  structure is
> @@ -2599,6 +2606,7 @@ \subsubsection{Configuring a Virtqueue}\label{sec:Virtio Transport Options / Vir
>          be16 num;
>          be64 driver;
>          be64 device;
> +        be32 indirect_num; /* since virtio-ccw rev. 3 */
>  };
>  \end{lstlisting}
>  
> @@ -2607,6 +2615,10 @@ \subsubsection{Configuring a Virtqueue}\label{sec:Virtio Transport Options / Vir
>  available area and used area for queue \field{index}, respectively. The actual
>  virtqueue size (number of allocated buffers) is transmitted in \field{num}.
>  
> +Since revision 3, \field{indirect_num} exists, which is supposed to reflect the
> +Queue Indirect Size (i.e. the maximum length of indirect descriptor tables)
> +supported by driver for this queue.

Similar to above:

"If revision 2 or lower is set, struct vq_info_block extends up to
\field{device}.

If revision 3 or higher is set, struct vq_info_block extends to
\field{indirect_num}. If VIRTIO_RING_F_INDIRECT_SIZE is negotiated,
the Queue Indirect Size (i.e. the maximum length of indirect descriptor
tables) supported by the driver for this queue is transmitted in
\field{indirect_num}."


> +
>  \devicenormative{\paragraph}{Configuring a Virtqueue}{Virtio Transport Options / Virtio over channel I/O / Device Initialization / Configuring a Virtqueue}
>  
>  \field{res0} is reserved and MUST be ignored by the device.

We should include a device normative statement:

"The device MUST NOT offer VIRTIO_RING_F_INDIRECT_SIZE, unless at least
revision 3 is set, and therefore \field{max_indirect_num} respectively
\field{indirect_num} are available."


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