[virtio-comment] Re: [PATCH v5 2/5] pci: Define id field

[Cornelia Huck <cohuck@redhat.com>]

On Thu, 27 Jun 2019 20:28:27 +0100
"Dr. David Alan Gilbert (git)" <dgilbert@redhat.com> wrote:

> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> For the virtio-fs device we require multiple large shared memory
> regions.  Differentiate these by an 'id' field in the base capability.
> 
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>  content.tex | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/content.tex b/content.tex
> index 6433226..41926c0 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -651,7 +651,8 @@ \subsection{Virtio Structure PCI Capabilities}\label{sec:Virtio Transport Option
>          u8 cap_len;     /* Generic PCI field: capability length */
>          u8 cfg_type;    /* Identifies the structure. */
>          u8 bar;         /* Where to find it. */
> -        u8 padding[3];  /* Pad to full dword. */
> +        u8 id;          /* Multiple capabilities of the same type */
> +        u8 padding[2];  /* Pad to full dword. */
>          le32 offset;    /* Offset within bar. */
>          le32 length;    /* Length of the structure, in bytes. */
>  };
> @@ -716,6 +717,11 @@ \subsection{Virtio Structure PCI Capabilities}\label{sec:Virtio Transport Option
>  
>          Any other value is reserved for future use.
>  
> +\item[\field{id}]
> +        Multiple capabilities of the same type can exist as long
> +        as they each have a unique \field{id}.  The specific

The requirement for id is new, isn't it? Shouldn't it rather be
optional?

> +        meaning of the field is different for each device type.
> +
>  \item[\field{offset}]
>          indicates where the structure begins relative to the base address associated
>          with the BAR.  The alignment requirements of \field{offset} are indicated

The current specification for cfg_type defines a kind of hierarchy for
capabilities of the same type (first one is preferred). We probably
need to be more explicit how id may interact with that (even if it is
device type specific).

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