Re: [Qemu-devel] [PATCH virtio 1/1] Add "Group Identifier" support to virtio PCI capabilities.

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

On Tue, Jun 19, 2018 at 11:32:28AM -0500, Venu Busireddy wrote:
> Add VIRTIO_PCI_CAP_GROUP_ID_CFG (Group Identifier) capability to the
> virtio PCI capabilities to allow for the grouping of devices.
> 
> Signed-off-by: Venu Busireddy <venu.busireddy@oracle.com>
> ---
>  content.tex | 43 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 43 insertions(+)
> 
> diff --git a/content.tex b/content.tex
> index 7a92cb1..7ea6267 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -599,6 +599,8 @@ The fields are interpreted as follows:
>  #define VIRTIO_PCI_CAP_DEVICE_CFG        4
>  /* PCI configuration access */
>  #define VIRTIO_PCI_CAP_PCI_CFG           5
> +/* Group Identifier */
> +#define VIRTIO_PCI_CAP_GROUP_ID_CFG      6
>  \end{lstlisting}
>  
>          Any other value is reserved for future use.
> @@ -997,6 +999,47 @@ address \field{cap.length} bytes within a BAR range
>  specified by some other Virtio Structure PCI Capability
>  of type other than \field{VIRTIO_PCI_CAP_PCI_CFG}.
>  
> +\subsubsection{Group Identifier capability}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Group Identifier capability}
> +
> +The VIRTIO_PCI_CAP_GROUP_ID_CFG capability provides means for grouping devices together.
> +
> +The capability is immediately followed by an identifier of arbitrary size as below:
> +
> +\begin{lstlisting}
> +struct virtio_pci_group_id_cap {
> +        struct virtio_pci_cap cap;
> +        u8 group_id[]; /* Group Identifier */
> +};
> +\end{lstlisting}
> +
> +The fields \field{cap.bar}, \field{cap.length}, \field{cap.offset}
> +and \field{group_id} are read-only for the driver.
> +
> +The specification does not impose any restrictions on the size or
> +structure of group_id[].

I think it must be a multiple of 4 in size, as is
standard for all capabilities.


> Vendors are free to declare this array as
> +large as needed, as long as the combined size of all capabilities can
> +be accommodated within the PCI configuration space.
> +
> +If there is enough room in the PCI configuration space to accommodate
> +the group identifier, the fields \field{cap.bar}, \field{cap.offset}
> +and \field{cap.length} should be set to 0.
> +
> +If there isn't enough room, some or all of the group identifier can be
> +presented in the BAR region, in which case the fields \field{cap.bar},
> +\field{cap.offset} and \field{cap.length} should be set appropriately.

And then how do you glue the two pieces?

> +
> +In either case, the field \field{cap.cap_len} indicates the length of
> +the group identifier information present in the configuration space
> +itself.

It seems like an overkill to me. Isn't it enough to have it in config
space? This would make comparisons easier.

> +
> +\devicenormative{\paragraph}{Group Identifier capability}{Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Group Identifier capability}
> +
> +The device MAY present the VIRTIO_PCI_CAP_GROUP_ID_CFG capability.
> +
> +\drivernormative{\paragraph}{Group Identifier capability}{Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Group Identifier capability}
> +
> +The driver MUST NOT write to group_id[] area or the BAR region.
> +
>  \subsubsection{Legacy Interfaces: A Note on PCI Device Layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Legacy Interfaces: A Note on PCI Device Layout}
>  
>  Transitional devices MUST present part of configuration