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

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On Thu, Dec 07, 2023 at 12:47:02PM +0000, Divin Raj wrote:
> 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@xxxxxxxxxxxxxx/>
> > > > > > > > 
> > > > > > > > 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@xxxxxxxxxx/
> > > > > > > [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

Arnaud brought this conversation to my attention.  I will address it in the
coming days.

> 
> > 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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