Re: [PATCH v6 0/4] Add a vhost RPMsg API

[Arnaud POULIQUEN <arnaud.pouliquen@st.com>]

Hi Guennadi,

On 9/18/20 7:44 AM, Guennadi Liakhovetski wrote:
> Hi Arnaud,
> 
> On Thu, Sep 17, 2020 at 05:21:02PM +0200, Arnaud POULIQUEN wrote:
>> Hi Guennadi,
>>
>>> -----Original Message-----
>>> From: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
>>> Sent: jeudi 17 septembre 2020 07:47
>>> To: Arnaud POULIQUEN <arnaud.pouliquen@st.com>
>>> Cc: kvm@vger.kernel.org; linux-remoteproc@vger.kernel.org;
>>> virtualization@lists.linux-foundation.org; sound-open-firmware@alsa-
>>> project.org; Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>; Liam
>>> Girdwood <liam.r.girdwood@linux.intel.com>; Michael S. Tsirkin
>>> <mst@redhat.com>; Jason Wang <jasowang@redhat.com>; Ohad Ben-Cohen
>>> <ohad@wizery.com>; Bjorn Andersson <bjorn.andersson@linaro.org>; Mathieu
>>> Poirier <mathieu.poirier@linaro.org>; Vincent Whitchurch
>>> <vincent.whitchurch@axis.com>
>>> Subject: Re: [PATCH v6 0/4] Add a vhost RPMsg API
>>>
>>> Hi Arnaud,
>>>
>>> On Tue, Sep 15, 2020 at 02:13:23PM +0200, Arnaud POULIQUEN wrote:
>>>> Hi  Guennadi,
>>>>
>>>> On 9/1/20 5:11 PM, Guennadi Liakhovetski wrote:
>>>>> Hi,
>>>>>
>>>>> Next update:
>>>>>
>>>>> v6:
>>>>> - rename include/linux/virtio_rpmsg.h ->
>>>>> include/linux/rpmsg/virtio.h
>>>>>
>>>>> v5:
>>>>> - don't hard-code message layout
>>>>>
>>>>> v4:
>>>>> - add endianness conversions to comply with the VirtIO standard
>>>>>
>>>>> v3:
>>>>> - address several checkpatch warnings
>>>>> - address comments from Mathieu Poirier
>>>>>
>>>>> v2:
>>>>> - update patch #5 with a correct vhost_dev_init() prototype
>>>>> - drop patch #6 - it depends on a different patch, that is currently
>>>>>   an RFC
>>>>> - address comments from Pierre-Louis Bossart:
>>>>>   * remove "default n" from Kconfig
>>>>>
>>>>> Linux supports RPMsg over VirtIO for "remote processor" / AMP use
>>>>> cases. It can however also be used for virtualisation scenarios,
>>>>> e.g. when using KVM to run Linux on both the host and the guests.
>>>>> This patch set adds a wrapper API to facilitate writing vhost
>>>>> drivers for such RPMsg-based solutions. The first use case is an
>>>>> audio DSP virtualisation project, currently under development, ready
>>>>> for review and submission, available at
>>>>> https://github.com/thesofproject/linux/pull/1501/commits
>>>>
>>>> Mathieu pointed me your series. On my side i proposed the rpmsg_ns_msg
>>>> service[1] that does not match with your implementation.
>>>> As i come late, i hope that i did not miss something in the history...
>>>> Don't hesitate to point me the discussions, if it is the case.
>>>
>>> Well, as you see, this is a v6 only of this patch set, and apart from it there have
>>> been several side discussions and patch sets.
>>>
>>>> Regarding your patchset, it is quite confusing for me. It seems that
>>>> you implement your own protocol on top of vhost forked from the RPMsg
>>> one.
>>>> But look to me that it is not the RPMsg protocol.
>>>
>>> I'm implementing a counterpart to the rpmsg protocol over VirtIO as initially
>>> implemented by drivers/rpmsg/virtio_rpmsg_bus.c for the "main CPU" (in case
>>> of remoteproc over VirtIO) or the guest side in case of Linux virtualisation.
>>> Since my implementation can talk to that driver, I don't think, that I'm inventing
>>> a new protocol. I'm adding support for the same protocol for the opposite side
>>> of the VirtIO divide.
>>
>> The main point I would like to highlight here is related to the use of the name "RPMsg"
>> more than how you implement your IPC protocol.
>> If It is a counterpart, it probably does not respect interface for RPMsg clients.
>> A good way to answer this, might be to respond to this question:
>> Is the rpmsg sample client[4] can be used on top of your vhost RPMsg implementation?
>> If the response is no, describe it as a RPMsg implementation could lead to confusion...
> 
> Sorry, I don't quite understand your logic. RPMsg is a communication protocol, not an 
> API. An RPMsg implementation has to be able to communicate with other compliant RPMsg 
> implementations, it doesn't have to provide any specific API. Am I missing anything?

You are right nothing is written in stone that compliance with the user RPMsg API defined
in the Linux Documentation [5] is mandatory.
IMO, as this API is defined in the Linux documentation [5] we should respect it, to ensure
one generic implementation. The RPMsg sample client[4] uses this user API, so seems to me
a good candidate to verify this. 

That's said, shall we multiple the RPMsg implementations in Linux with several APIs,
With the risk to make the RPMsg clients devices dependent on these implementations?
That could lead to complex code or duplications...

I'm not the right person to answer, Bjorn and Mathieu are.

[5] https://elixir.bootlin.com/linux/v5.8.10/source/Documentation/rpmsg.txt#L66

Thanks,
Arnaud

  
> 
> Thanks
> Guennadi
> 
>> [4] https://elixir.bootlin.com/linux/v5.9-rc5/source/samples/rpmsg/rpmsg_client_sample.c
>>
>> Regards,
>> Arnaud
>>
>>>
>>>> So i would be agree with Vincent[2] which proposed to switch on a
>>>> RPMsg API and creating a vhost rpmsg device. This is also proposed in
>>>> the "Enhance VHOST to enable SoC-to-SoC communication" RFC[3].
>>>> Do you think that this alternative could match with your need?
>>>
>>> As I replied to Vincent, I understand his proposal and the approach taken in the
>>> series [3], but I'm not sure I agree, that adding yet another virtual device /
>>> driver layer on the vhost side is a good idea. As far as I understand adding new
>>> completely virtual devices isn't considered to be a good practice in the kernel.
>>> Currently vhost is just a passive "library"
>>> and my vhost-rpmsg support keeps it that way. Not sure I'm in favour of
>>> converting vhost to a virtual device infrastructure.
>>>
>>> Thanks for pointing me out at [3], I should have a better look at it.
>>>
>>> Thanks
>>> Guennadi
>>>
>>>> [1].
>>>> https://patchwork.kernel.org/project/linux-remoteproc/list/?series=338
>>>> 335 [2].
>>>> https://www.spinics.net/lists/linux-virtualization/msg44195.html
>>>> [3]. https://www.spinics.net/lists/linux-remoteproc/msg06634.html
>>>>
>>>> Thanks,
>>>> Arnaud
>>>>
>>>>>
>>>>> Thanks
>>>>> Guennadi
>>>>>
>>>>> Guennadi Liakhovetski (4):
>>>>>   vhost: convert VHOST_VSOCK_SET_RUNNING to a generic ioctl
>>>>>   rpmsg: move common structures and defines to headers
>>>>>   rpmsg: update documentation
>>>>>   vhost: add an RPMsg API
>>>>>
>>>>>  Documentation/rpmsg.txt          |   6 +-
>>>>>  drivers/rpmsg/virtio_rpmsg_bus.c |  78 +------
>>>>>  drivers/vhost/Kconfig            |   7 +
>>>>>  drivers/vhost/Makefile           |   3 +
>>>>>  drivers/vhost/rpmsg.c            | 373 +++++++++++++++++++++++++++++++
>>>>>  drivers/vhost/vhost_rpmsg.h      |  74 ++++++
>>>>>  include/linux/rpmsg/virtio.h     |  83 +++++++
>>>>>  include/uapi/linux/rpmsg.h       |   3 +
>>>>>  include/uapi/linux/vhost.h       |   4 +-
>>>>>  9 files changed, 551 insertions(+), 80 deletions(-)  create mode
>>>>> 100644 drivers/vhost/rpmsg.c  create mode 100644
>>>>> drivers/vhost/vhost_rpmsg.h  create mode 100644
>>>>> include/linux/rpmsg/virtio.h
>>>>>