[virtio-comment] Re: [PATCH v3 2/3] transport-pci: Introduce legacy registers access commands

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

On Fri, Jun 02, 2023 at 11:36:03PM +0300, Parav Pandit wrote:
> +\subsubsection{Legacy Device Registers Read Command}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / Legacy Interfaces: SR-IOV VFs Registers Access / Legacy Common Configuration Registers Read Command}
> +
> +The Legacy Device Registers Read Command follows \field{struct virtio_admin_cmd}.
> +This command reads legacy device specific registers of a member VF device.
> +The driver sets command \field{opcode} to VIRTIO_ADMIN_CMD_LD_REG_READ.
> +
> +The command VIRTIO_ADMIN_CMD_LD_REG_READ uses following listed structure for
> +\field{command_specific_data}:
> +
> +\begin{lstlisting}
> +struct virtio_admin_cmd_ld_reg_rd_data {
> +	u8 offset; /* Starting byte offset of the register to read */
> +};
> +\end{lstlisting}
> +
> +When command completes successfully, \field{command_specific_result}
> +uses following listed structure:
> +
> +\begin{lstlisting}
> +struct virtio_admin_cmd_ld_reg_rd_result {
> +	u8 registers[];
> +};
> +\end{lstlisting}
> +
> +\subsubsection{Legacy Queue Notify Offset Query Command}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / Legacy Interfaces: SR-IOV VFs Registers Access / Legacy Queue Notify Offset Query Command}
> +
> +This command returns the notify offset of the member VF for virtqueue
> +driver notifications. This command follows \field{struct virtio_admin_cmd}.
> +The driver sets command \field{opcode} to VIRTIO_ADMIN_CMD_LQ_NOTIFY_QUERY.
> +There is no command specific data for this command.
> +
> +When command completes successfully, \field{command_specific_result}
> +uses following listed structure:
> +
> +\begin{lstlisting}
> +struct virtio_admin_cmd_lq_notify_query_result {
> +	u8 bar; /* PCI BAR number of the member VF */
> +	u8 reserved[7];
> +	le64 offset; /* Byte offset within the BAR */
> +};
> +\end{lstlisting}
> +
> +The driver that may use the driver notifications region of the VF device
> +returned in this result likely attain higher performance or the drier may use
> +the VIRTIO_ADMIN_CMD_LREG_WRITE command.

Obtain I guess ... but how? There's no explanation.


> +
> +\begin{note}
> +The device and driver must encode and decode legacy device specific registers
> +using little endian format. Per PCI VF device level big endian format support
> +is left for the future.
> +\end{note}
> +
> +\begin{note}
> +The PCI VF device should not use PCI BAR 0 when it prefers to support
> +legacy interface registers access using its group owner PF. This enables
> +hypervisor software to operate with least complexities to compose a legacy
> +interface I/O space BAR and passthrough other PCI BARs and PCI device
> +capabilities to the guest virtual machine without any translation.
> +\end{note}

Is this related to this last command somehow? what does it mean
for PCI VF device to use a BAR? not use a BAR? Prefer what to what?

> -- 
> 2.26.2


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