Re: [PATCH RFC v2 1/1] virtio-msg: Add virtio-msg, a message based virtio transport layer

Hello Matias,

On 9/10/25 9:39 AM, Matias Ezequiel Vara Larsen wrote:
> Hello, 
> 
> Thanks for submission. I am still reading this RFC. I left some minor
> comments below though.
> 

I will incorpertate all your editorial comments you made.  Thanks for looking.

I will respond to your questions below ...

> On Fri, Jun 20, 2025 at 06:44:26PM -0400, Bill Mills wrote:
>> Add a new transport layer that is based on messages.
>>
>> This transport layer still uses virtqueues as the other transport layers do
>> but implements transport layer operations by sending and receiving messages
>> instead of the "MMR" reads and writes used in virtio-mmio and virtio-pci.
> 
> I guess virtqueues are still on a region of memory shared between the
> device and the driver and a msgs mechanism is not required to access it. 
> 

Yes, the memory used for virtqueues and for buffers in those virtqueues is
shared between driver and device somehow.

In my OSS NA presentation I disucss various memory models as I think about
them anyway.

See slide 11 here:
https://static.sched.com/hosted_files/ossna2025/a6/Virtio-msg-OSS-NA-2025-v1.1.pdf

[snip]

>> +The fields in this header have the following usage:
>> +\begin{itemize}
>> +  \item \field{type}:
>> +    \begin{itemize}
>> +      \item Bit[0]: Identifies if a message is a request (0) or a response
>> +          to a request (1).
>> +      \item Bit[1]: Identifies if a message is a Transport Message (0) or a
>> +          Bus Message (1).
>> +      \item Bit[2-7] Are reserved for future use and must be zero.
>> +    \end{itemize}
>> +  \item \field{msg_id}:
>> +    Uniquely identifies which message definition applies (e.g., GET_DEVICES,
>> +    GET_DEVICE_FEATURES, SET_CONFIG). The specific range or enumeration of types is
>> +    defined in sections \ref{sec:Virtio Transport Options / Virtio Over Messages / Transport Messages}
>> +    and \ref{sec:Virtio Transport Options / Virtio Over Messages / Bus Messages}.
>> +  \item \field{dev_num}:
>> +     Identifies the Device Number the message is targeting or is coming from for
>> +     Transport Message and must be zero of Bus messages.
> 
> I imagine that the `dev_num` is the mechanism that the bus uses to find
> the correct destination for a msg both when the driver or the device
> sends it, right?.
> 

That is correct.  A bus can have multiple devices and dev_num (a simple
ordinal index) defines which one.  The ordinal space may be sparse.

Thanks again,
Bill