Re: [PATCH 2/2] Add common configuration field "queue_indirect_size"

[Stefan Hajnoczi <stefanha@redhat.com>]

On Tue, Dec 14, 2021 at 04:13:17PM +0100, Christian Schoenebeck wrote:
> This new common configuration field allows to negotiate a more fine
> graded maximum lenght of indirect descriptor chains.
> 
> Fixes: https://github.com/oasis-tcs/virtio-spec/issues/122
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> ---
>  content.tex    | 25 ++++++++++++++++++++++++-
>  split-ring.tex |  3 +++
>  2 files changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/content.tex b/content.tex
> index 0aa4842..e3cfcae 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -859,6 +859,7 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
>          le64 queue_driver;              /* read-write */
>          le64 queue_device;              /* read-write */
>          le16 queue_notify_data;         /* read-only for driver */
> +        le32 queue_indirect_size;       /* read-write */

I see no other unaligned fields in this struct, so I think a le16
padding field is needed for safety:

           le64 queue_device;              /* read-write */
           le16 queue_notify_data;         /* read-only for driver */
  +        le16 reserved;                  /* no access */
  +        le32 queue_indirect_size;       /* read-write */

>  };
>  \end{lstlisting}
>  
> @@ -938,6 +939,16 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
>          may benefit from providing another value, for example an internal virtqueue
>          identifier, or an internal offset related to the virtqueue number.
>          \end{note}
> +
> +\item[\field{queue_indirect_size}]
> +        This field is used to negotiate the maximum amount of descriptors per
> +        vring slot as in \ref{sec:Basic Facilities of a Virtio Device /
> +        Virtqueues / The Virtqueue Descriptor Table / Indirect Descriptors} if
> +        and only if the VIRTIO_RING_F_INDIRECT_SIZE feature has been negotiated.
> +
> +        The device specifies its maximum supported number of descriptors per
> +        vring slot. If the driver requires fewer descriptors, it writes its
> +        lower value to inform the device of the reduced resource requirements.

"vring slot" is a little vague, it means "indirect descriptor
table"? The two paragraphs could use "maximum number of descriptors per
indirect descriptor table" instead of referring to vring slots to imply
indirect descriptor tables.

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