[virtio-comment] Re: [virtio-dev] [PATCH v4 2/3] shared memory: Define PCI capability

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

On Wed, Mar 20, 2019 at 12:07:39PM +0000, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> Define the PCI capability used for enumerating shared memory regions.
> 
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> Reviewed-by: Cornelia Huck <cohuck@redhat.com>
> ---
>  content.tex | 37 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 37 insertions(+)
> 
> diff --git a/content.tex b/content.tex
> index 8cd1a38..ddbf949 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -688,6 +688,8 @@ The fields are interpreted as follows:
>  #define VIRTIO_PCI_CAP_DEVICE_CFG        4
>  /* PCI configuration access */
>  #define VIRTIO_PCI_CAP_PCI_CFG           5
> +/* Shared memory region */
> +#define VIRTIO_PCI_CAP_SHARED_MEMORY_CFG 8
>  \end{lstlisting}
>  
>          Any other value is reserved for future use.
> @@ -1052,6 +1054,41 @@ any device type which has a device-specific configuration.
>  
>  The \field{offset} for the device-specific configuration MUST be 4-byte aligned.
>  
> +\subsubsection{Shared memory capability}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Shared memory capability}
> +
> +Shared memory regions \ref{sec:Basic Facilities of a Virtio
> +Device / Shared Memory Regions} are enumerated on the PCI transport
> +as a sequence of VIRTIO_PCI_CAP_SHARED_MEMORY_CFG capabilities, one per region.
> +
> +The capability is immediately followed by an additional field like so:
> +
> +\begin{lstlisting}
> +struct virtio_pci_shm_cap {
> +        struct virtio_pci_cap cap;
> +        u32 offset_hi;
> +        u32 length_hi;
> +        u8  id;

Pls pad, pci capabilities must be a multiple of 4 bytes.
BTW virtio_pci_cap has 3 bytes of padding.
We can stick the id there maybe?

> +};
> +\end{lstlisting}
> +
> +Given that the \field{cap.length} and \field{cap.offset} fields
> +are only 32 bit, the additional \field{offset_hi} and \field {length_hi}
> +fields provide the most significant 32 bits of a total 64 bit offset and
> +length within the bar specified by \field{cap.bar}.  \field{id}
> +uniquely identifies the given shared memory region using a device
> +specific identifier.

I didn't realize we will ever need so much space.  I think I prefer
moving offset_hi/length_hi to a general virtio_pci_cap64 that others can
reuse.


> +
> +While most capabilities can not be repeated,

I'd drop this part.

> multiple
> +VIRTIO_PCI_CAP_SHARED_MEMORY_CFG capabilities are allowed as long
> +as the \field{id} is unique.
> +
> +\devicenormative{\paragraph}{Device-specific configuration}{Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Shared memory capability}
> +
> +The region defined by the combination of the offsets and length
> +fields 

let's do \field{cap.length} \field{cap.offset} etc.

>MUST be contained within the declared bar.
> +
> +The \field{id} MUST be unique for any one device instance.
> +
>  \subsubsection{PCI configuration access capability}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / PCI configuration access capability}
>  
>  The VIRTIO_PCI_CAP_PCI_CFG capability
> -- 
> 2.20.1
> 
> 
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