[virtio-dev] Re: [PATCH v11 08/10] admin: command list discovery

[Parav Pandit <parav@nvidia.com>]

On 4/3/2023 11:03 AM, Michael S. Tsirkin wrote:
> Add commands to find out which commands does each group support,
> as well as enable their use by driver.
> This will be especially useful once we have multiple group types.
> 
> An alternative is per-type VQs. This is possible but will
> require more per-transport work. Discovery through the vq
> helps keep things contained.
> 
> e.g. lack of support for some command can switch to a legacy mode
>
Above description about legacy is not clear. what is legacy mode in 
context of admin vq?
> note that commands are expected to be avolved by adding new
s/to be avolved/to evolve/

> fields to command specific data at the tail, so
> we generally do not need feature bits for compatibility.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> 
> ---
> dropped S.O.B by Max
> explain in commit log how commands will evolve, comment by Stefan
> replace will use with is capable of use, comment by Stefan
> typo fix reported by David and Lingshan
> rename cmds to cmd_opcodes suggested by Jiri
> list group_type explicitly in command description suggested by Jiri
> clarify how can device not support some commands
> always say group administration commands, comment by Jiri
> ---
>   admin.tex | 102 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 101 insertions(+), 1 deletion(-)
> 
> diff --git a/admin.tex b/admin.tex
> index f201bcd..9552797 100644
> --- a/admin.tex
> +++ b/admin.tex
> @@ -113,7 +113,9 @@ \subsection{Group administration commands}\label{sec:Basic Facilities of a Virti
>   \hline
>   opcode & Name & Command Description \\
>   \hline \hline
> -0x0000 - 0x7FFF & - & Commands using \field{struct virtio_admin_cmd}    \\
> +0x0000 & VIRTIO_ADMIN_CMD_LIST_QUERY & Provides to driver list of commands supported for this group type    \\
> +0x0001 & VIRTIO_ADMIN_CMD_LIST_USE & Provides to device list of commands used for this group type \\
> +0x0002 - 0x7FFF & - & Commands using \field{struct virtio_admin_cmd}    \\
>   \hline
>   0x8000 - 0xFFFF & - & Reserved for future commands (possibly using a different structure)    \\
>   \hline
> @@ -183,6 +185,104 @@ \subsection{Group administration commands}\label{sec:Basic Facilities of a Virti
>   depends on these structures and is described separately or is
>   implicit in the structure description.
>   
> +Before sending any group administration commands to the device, the driver
> +needs to communicate to the device which commands it is going to
> +use. Initially (after reset), only two commands are assumed to be used:
> +VIRTIO_ADMIN_CMD_LIST_QUERY and VIRTIO_ADMIN_CMD_LIST_USE.
> +
> +Before sending any other commands for any member of a specific group to
> +the device, the driver queries the supported commands via
> +VIRTIO_ADMIN_CMD_LIST_QUERY and sends the commands it is
> +capable of using via VIRTIO_ADMIN_CMD_LIST_USE.
> +
> +Commands VIRTIO_ADMIN_CMD_LIST_QUERY and
> +VIRTIO_ADMIN_CMD_LIST_USE
> +both use the following structure describing the
> +command opcodes:
> +
> +\begin{lstlisting}
> +struct virtio_admin_cmd_list {
> +       /* Indicates which of the below fields were returned
> +       le64 device_admin_cmd_opcodes[];
> +};
> +\end{lstlisting}
> +
> +This structure is an array of 64 bit values in little-endian byte
> +order, in which a bit is set if the specific command opcode
> +is supported. Thus, \field{device_admin_cmd_opcodes[0]} refers to the
> +first 64-bit value in this array corresponding to opcodes 0 to
> +63, \field{device_admin_cmd_opcodes[1]} is the second 64-bit value
> +corresponding to opcodes 64 to 127, etc.
> +For example, the array of size 2 including
> +the values 0x3 in \field{device_admin_cmd_opcodes[0]}
> +and 0x1 in \field{device_admin_cmd_opcodes[1]} indicates that only
> +opcodes 0, 1 and 64 are supported.
> +The length of the array depends on the supported opcodes - it is
> +large enough to include bits set for all supported opcodes,
> +that is the length can be calculated by starting with the largest
> +supported opcode adding one, dividing by 64 and rounding up.
> +In other words, for
> +VIRTIO_ADMIN_CMD_LIST_QUERY and VIRTIO_ADMIN_CMD_LIST_USE the
> +length of \field{command_specific_result} and
> +\field{command_specific_data} respectively will be
> +$DIV_ROUND_UP(max_cmd, 64) * 8$ where DIV_ROUND_UP is integer division
> +with round up and \field{max_cmd} is the largest available command opcode.
> +
> +The array is also allowed to be larger and to additionally
> +include an arbitrary number of all-zero entries.
> +
> +Accordingly, bits 0 and 1 corresponding to opcode 0
> +(VIRTIO_ADMIN_CMD_LIST_QUERY) and 1
> +(VIRTIO_ADMIN_CMD_LIST_USE) are
> +always set in \field{device_admin_cmd_opcodes[0]} returned by VIRTIO_ADMIN_CMD_LIST_QUERY.
> +
> +For the command VIRTIO_ADMIN_CMD_LIST_QUERY, \field{opcode} is set to 0x0.
> +The \field{group_member_id} is unused. It is set to zero by driver.
> +This command has no command specific data.
> +The device, upon success, returns a result in
> +\field{command_specific_result} in the format
> +\field{struct virtio_admin_cmd_list} describing the
> +list of group administration commands supported for the group type
> +specified by \field{group_type}.
> +
> +For the command VIRTIO_ADMIN_CMD_LIST_USE, \field{opcode}
> +is set to 0x1.
> +The \field{group_member_id} is unused. It is set to zero by driver.
> +The \field{command_specific_data} is in the format
> +\field{struct virtio_admin_cmd_list} describing the
> +list of group administration commands used by the driver
> +with the group type specified by \field{group_type}.
> +
> +This command has no command specific result.
> +
> +The driver issues the command VIRTIO_ADMIN_CMD_LIST_QUERY to
> +query the list of commands valid for this group and before sending
> +any commands for any member of a group.
> +
> +The driver then enables use of some of the opcodes by sending to
> +the device the command VIRTIO_ADMIN_CMD_LIST_USE with a subset
> +of the list returned by VIRTIO_ADMIN_CMD_LIST_QUERY that is
> +both understood and used by the driver.
> +
We need to clarify that if LIST_USE command can be called only one time 
or multiple times.

For example first time for limited functionality, it enabled 10 commands 
specific to SR-IOV.
Later when user wants to use SIOV, it enables additional SIOV specific 
command set.

> +If the device supports the command list used by the driver, the
> +device completes the command with status VIRTIO_ADMIN_STATUS_OK.
> +If the device does not support the command list
> +(for example, if the driver is not capable to use
> +some required commands), the device
> +completes the command with status
> +VIRTIO_ADMIN_STATUS_INVALID_FIELD.
> +
> +Note: drivers are assumed not to set bits in
> +device_admin_cmd_opcodes
> +if they are not familiar with how the command opcode
> +is used, since devices could have dependencies between
> +command opcodes.
> +
Can you please change to use the singular notion for device and driver 
in the note like rest of the above description?

> +It is assumed that all members in a group support and are used
> +with the same list of commands. However, for owner devices
> +supporting multiple group types, the list of supported commands
> +might differ between different group types.
> +
>   \section{Administration Virtqueues}\label{sec:Basic Facilities of a Virtio Device / Administration Virtqueues}
>   
>   An administration virtqueue of an owner device is used to submit

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