[virtio-dev] Re: [PATCH v3 1/2] virtio-net: Describe dev cfg fields read only

[Cornelia Huck <cohuck@redhat.com>]

On Fri, Feb 17 2023, Parav Pandit <parav@nvidia.com> wrote:

> Device configuration fields are read only. Avoid duplicating this
> description for multiple fields.
>
> Instead describe it one time and do it in the driver requirements
> section.
>
> Fixes: https://github.com/oasis-tcs/virtio-spec/issues/161
> Signed-off-by: Parav Pandit <parav@nvidia.com>
> ---
> changelog:
> v2->v3:
> - split as new patch
> ---
>  device-types/net/description.tex | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/device-types/net/description.tex b/device-types/net/description.tex
> index a197e1a..81e1135 100644
> --- a/device-types/net/description.tex
> +++ b/device-types/net/description.tex
> @@ -156,10 +156,10 @@ \subsubsection{Legacy Interface: Feature bits}\label{sec:Device Types / Network
>  \subsection{Device configuration layout}\label{sec:Device Types / Network Device / Device configuration layout}
>  \label{sec:Device Types / Block Device / Feature bits / Device configuration layout}
>  
> -Device configuration fields are listed below, they are read-only for a driver. The \field{mac} address field
> +Device configuration fields are listed below. The \field{mac} address field

I would not remove this here, as I don't think we should move a simple
statement into the conformance section (see below.) It does makes sense
to remove the duplicate read-only annotations from the individual
fields.

>  always exists (though is only valid if VIRTIO_NET_F_MAC is set), and
>  \field{status} only exists if VIRTIO_NET_F_STATUS is set. Two
> -read-only bits (for the driver) are currently defined for the status field:
> +bits (for the driver) are currently defined for the status field:

What does "bits (for the driver)" mean? It made sense together with
"read-only", but I would drop "(for the driver)" as well.

>  VIRTIO_NET_S_LINK_UP and VIRTIO_NET_S_ANNOUNCE.
>  
>  \begin{lstlisting}
> @@ -167,14 +167,14 @@ \subsection{Device configuration layout}\label{sec:Device Types / Network Device
>  #define VIRTIO_NET_S_ANNOUNCE    2
>  \end{lstlisting}
>  
> -The following driver-read-only field, \field{max_virtqueue_pairs} only exists if
> +The following field, \field{max_virtqueue_pairs} only exists if
>  VIRTIO_NET_F_MQ or VIRTIO_NET_F_RSS is set. This field specifies the maximum number
>  of each of transmit and receive virtqueues (receiveq1\ldots receiveqN
>  and transmitq1\ldots transmitqN respectively) that can be configured once at least one of these features
>  is negotiated.
>  
> -The following driver-read-only field, \field{mtu} only exists if
> -VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the driver to
> +The following field, \field{mtu} only exists if VIRTIO_NET_F_MTU
> +is set. This field specifies the maximum MTU for the driver to
>  use.
>  
>  The following two fields, \field{speed} and \field{duplex}, only
> @@ -261,6 +261,8 @@ \subsection{Device configuration layout}\label{sec:Device Types / Network Device
>  
>  \drivernormative{\subsubsection}{Device configuration layout}{Device Types / Network Device / Device configuration layout}
>  
> +All the device configuration fields are read-only for the driver.

Not sure if this makes a good normative clause, I would rather give the
driver something actionable:

"A driver SHOULD NOT try to write to any of the device configuration
fields."

> +
>  A driver SHOULD negotiate VIRTIO_NET_F_MAC if the device offers it.
>  If the driver negotiates the VIRTIO_NET_F_MAC feature, the driver MUST set
>  the physical address of the NIC to \field{mac}.  Otherwise, it SHOULD


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