Re: [Discussion]: Enhance virtio rpmsg bus driver buffer allocation

[Divin Raj <divin.raj@arm.com>]

On 11/28/23 1:45 PM, Arnaud POULIQUEN wrote:
>
>
> On 11/28/23 12:19, Divin Raj wrote:
>> On 11/28/23 8:34 AM, Arnaud POULIQUEN wrote:
>>>
>>>
>>> On 11/24/23 17:45, Divin Raj wrote:
>>>> Hi Arnaud,
>>>> Please find my comments inline.
>>>>
>>>> On 11/20/23 10:14 AM, Arnaud POULIQUEN wrote:
>>>>> Hi Divin,
>>>>>
>>>>> On 11/17/23 23:24, Divin Raj wrote:
>>>>>> On 10/23/23 11:44 AM, Divin Raj wrote:
>>>>>>> Hello all,
>>>>>>>
>>>>>>> I am reaching out with reference to the patch discussed here: Enhanced
>>>>>>> virtio rpmsg bus driver buffer allocation.
>>>>>>> <https://lore.kernel.org/all/CAH2Cfb-sv3SAL8bcczC-Dc3_r58MYZCS7s7zGtn1Qfo3mmBqVg@mail.gmail.com/>
>>>>>>>
>>>>>>> I've been keenly following the developments around enhancing buffer
>>>>>>> allocation strategies, especially those focused on dynamic buffer sizing
>>>>>>> and the considerations for systems under varying memory constraints.This
>>>>>>> work is highly relevant to several projects I am involved in, and I am
>>>>>>> quite interested in its progression. May I kindly request an update on
>>>>>>> the current phase of these initiatives? Additionally, I am eager to know
>>>>>>> if there would be an opportunity for me to contribute to enhancing the
>>>>>>> patch, possibly by working on improvements or assisting in verification
>>>>>>> processes.
>>>>>>>
>>>>>>> Furthermore, if there are any condensed resources, summaries, or
>>>>>>> specific threads that encapsulate recent advancements or discussions on
>>>>>>> this topic, I would be grateful to receive directions to them.
>>>>>>>
>>>>>>> I appreciate everyone's dedicated efforts and invaluable contributions
>>>>>>> to this area of development. Looking forward to the updates.
>>>>>>>
>>>>>>> Regards Divin
>>>>>>>
>>>>>> Hello Linux Community,
>>>>>>
>>>>>> In one of our internal projects, we encountered a challenge with RPMSG
>>>>>> buffer allocation. Our goal is to optimize memory allocation for an
>>>>>> out-of-tree RPMSG Ethernet device driver using virtio. This is to ensure
>>>>>> support for packet sizes matching the standard MTU (Maximum Transmission
>>>>>> Unit) size of 1500 bytes.
>>>>>>
>>>>>> To mitigate this issue, There are few possible solutions:
>>>>>>
>>>>>> 1. Configure buffer size and number through Kconfig.
>>>>>> 2. Permit the firmware creator to determine the most suitable value from
>>>>>>      the resource table.
>>>>>> 3. Enable independent configurations on both ends. This approach would
>>>>>> support both dynamic and fixed buffer configurations using a generic
>>>>>> allocator.
>>>>>>
>>>>>> Reference:
>>>>>>
>>>>>> [1]:
>>>>>> https://lore.kernel.org/all/1548949280-31794-4-git-send-email-xiaoxiang@xiaomi.com/
>>>>>> [2]: https://lore.kernel.org/all/20190701061353.GE1263@builder/
>>>>>>
>>>>>>
>>>>>> Draft Design Overview:
>>>>>>
>>>>>> Based on the reference patch and the discussions, we have outlined the
>>>>>> following key points for the belw design:
>>>>>>
>>>>>> 1. Assure compatibility, enabling both Linux and the remote system to
>>>>>> interchangeably transmit and receive messages, irrespective of size.
>>>>>> 2. For systems with constrained shared memory:
>>>>>> Systems with small, shared memory, we need to deal with a
>>>>>> limited/optimized memory chunk. To avoid memory fragmentation, the
>>>>>> allocator should have a pre-reserved buffer pool
>>>>>> 3. The implementation should ensure that the remote side does not
>>>>>> receive messages based on its allocation parameters.
>>>>>>
>>>>>> do you think it could make sense?
>>>>>>
>>>>>> High level view:
>>>>>> +------------------+                               +------------------+
>>>>>> |                  |                               |                  |
>>>>>> |      Linux       |                               |      Remote      |
>>>>>> |                  |                               |                  |
>>>>>> |   +----------+   |       +-----------------+     |   +----------+   |
>>>>>> |   |   RPMSG  |   | <---> | Buffer Allocator|<--->|   | RPMSG    |   |
>>>>>> |   +----------+   |       | (Dynamic/Static)|     |   +----------+   |
>>>>>> |                  |       +-----------------+     |                  |
>>>>>> +------------------+                               +------------------+
>>>>>>
>>>>>>
>>>>>> Detailed view:
>>>>>>
>>>>>>                      +-------------------------+
>>>>>>                      |  Message Creation       |
>>>>>>                      |  (Both Linux/Remote)    |
>>>>>>                      +------------+------------+
>>>>>>                                   |
>>>>>>                                   v
>>>>>>                      +-------------------------+
>>>>>>                      | Determine the allocation|
>>>>>>                      | strategy                |
>>>>>>                      +------------+------------+
>>>>>>                                   |
>>>>>>                    +--------------+--------------+
>>>>>>                    |                             |
>>>>>> +-------------------------------+  +-------------------------------+
>>>>>> | Dynamic allocation            |  | Static allocation             |
>>>>>> | (Buffer allocator allocates   |  | (Pre-reserved memory          |
>>>>>> | memory space as needed,       |  | space)                        |
>>>>>> | based on the current          |  |                               |
>>>>>> | message requirement )         |  |                               |
>>>>>> +-------------------------------+  +-------------------------------+
>>>>>
>>>>> Do you have a proposal for dynamic allocation?
>>>>>
>>>>> RPMSG is based on the virtio protocol. The virtio driver in the Linux kernel
>>>>> is responsible for allocating buffers for the virtio device on the remote
>>>>> processor.
>>>>>
>>>>> In the current implementation (static allocation) the Linux
>>>>> kernel allocates predefined buffers for the remote processor.
>>>>>
>>>>> How would you manage the fact that the sender allocates its own buffers and
>>>>> references
>>>>> them in the vring descriptor? This would require each core to have
>>>>> a dual role, right?
>>>>> - a virtio driver role on its TX vring
>>>>> - a virtio device role on its RX vring."
>>>>>
>>>> I'm unsure if a dual role is feasible under the Virtio specification.
>>>
>>> At least, it does not seem to align with the philosophy of VirtIO.
>>>
>>>
>>>> However, would it make sense to set the size of the outbuf based on the
>>>> Maximum Transmission Unit (MTU) size that is supported? Additionally,
>>>> the size of the inbuf could be set by the firmware, suggesting that it
>>>> should be derived from the resource table. With this approach, I believe
>>>> the sender can decide the maximum size.
>>>
>>> It is not clear to me what your proposal is.
>>> Are you speaking about a pre-allocated buffers as proposed in [1],
>>> or are you speaking about dynamic allocation of the RPMsg in a pool?
>>
>> we are at the initial phase of this investigation. As we previously
>> discussed, option 3 is not feasible in accordance with the virtio
>> specification.The above proposed solution aligns with [1], suggesting
>> preallocated in_buf and out_buf, with sizes determined from the resource
>> table and MTU. By allowing Linux to decide the out_buf size and the
>> remote to decide the in_buf size, I believe we can avoid conflicts. If
>> everyone agrees on a common idea, then it would be a good starting point
>
> Thanks for the clarification. It seems reasonable to me to start with a
> pre-allocated buffer with a fixed size specified by the remote firmware.
>
> Bjorn, Mathieu,
> Please, could you share you point of view on the topic?
>
+ Bjorn
+ Mathieu

> Thanks,
> Arnaud
>
>>
>> Regards
>> Divin
>>
>>> Regards,
>>> Arnaud
>>>
>>>>
>>>> Regards
>>>> Divin
>>>>
>>>>>
>>>>> Regards,
>>>>> Arnaud
>>>>>
>>>>
>>>>>
>>>>>>
>>>>>> We would greatly appreciate any feedback, suggestions, or improvements
>>>>>> you could provide.
>>>>>>
>>>>>> Thank you for your time and consideration.
>>>>>>
>>>>>> Regards
>>>>>> Divin

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