Re: [PATCH v1 0/4] virtio-msg transport layer

Bertrand Marquis
2026-02-25T09:36:42Z

Hi Michael and Parav,

> On 25 Feb 2026, at 10:22, Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Wed, Feb 25, 2026 at 09:18:58AM +0000, Parav Pandit wrote:
>>
>>> From: Michael S. Tsirkin <mst@redhat.com>
>>> Sent: 25 February 2026 12:55 PM
>>>
>>> On Wed, Feb 25, 2026 at 05:09:45AM +0000, Parav Pandit wrote:
>>>>
>>>>
>>>>> From: Michael S. Tsirkin <mst@redhat.com>
>>>>> Sent: 20 February 2026 03:33 PM
>>>>>
>>>>> On Fri, Feb 20, 2026 at 06:13:55AM +0000, Parav Pandit wrote:
>>>>>>>> 4. I dont find the transport messages to read and write to the driver memory supplied in VIRTIO_MSG_SET_VQUEUE addresses to
>>>>> operate
>>>>>>> the virtqueues.
>>>>>>>> Dont we need VIRTIO_MEM_READ, VIRTIO_MEM_WRITE request and response?
>>>>>>>
>>>>>>> surely this can be an optional transport feature bit.
>>>>>>>
>>>>>> How is this optional?
>>>>>> How can one implement a transport without defining the basic data transfer semantics?
>>>>>> For example for TCP transporting, driver side OS is implementing header format foo, and device side is using header format bar.
>>>>>> How does it work?
>>>>>
>>>>> I'm not sure what do foo/bar refer to, or what TCP transporting means.
>>>> This proposal is subset of past proposal of [1].
>>>>
>>>> Proposal [1] covered transporting virtio control and data operations using something other than MMIO and PCI.
>>>> And current proposal is similar, except that it didn't define the transport binding at all for the specific bus.
>>>> It is only a partial 'control-transport'.
>>>>
>>>> [1] https://yhbt.net/lore/virtio-comment/20231023104647.290759-2-pizhenwei@bytedance.com/
>>>>
>>>> So foo and bar are the definitions I expect as listed in the patch-5 of [1].
>>>> If it has to be done by the bus, lets write up this as 'control-transport'.
>>>>
>>>>> The simplest way to do TCP on top of virtio is to layer it above virtio
>>>>> net. That uses VQs for data transfers.
>>>>>
>>>> The intention of this proposal is not to do TCP on top of virtio.
>>>> The intention of this proposal is to do virtio on transports other than MMIO, PCI and channel.
>>>> Such a transport can be anything - not defined in virtio spec.
>>>
>>>> It could be FFA, some two SoC as written in cover letter example, or it can be something else such as TCP or UDP or vsock or whatever else.
>>>
>>> I feel this "anything" is simply too broad a requirement.
>>> I did not see any demand for virtio over TCP.
>>> And, making it work with existing drivers will be a mess.
>> Why would it be a mess? Because of load/store semantics?
>> If so, would the message layer also faces the same challenge?
>
> not sure what "the message layer" is.
>
>>> We can scope this for buses that can do DMA for now.
>> That looks reasonable good start.
>> However, "Appendix C. Creating New Transports" needs modification.
>>
>> Following two likely needs to stay outside of the "control transport".
>>
>> A transport provides a mechanism for the device to send device notifications to the driver, such as used
>> buffer notifications.
>> A transport provides a mechanism for the driver to send driver notifications to the device, such as available
>> buffer notifications.
>
>
> Well existing transports sure do include these.
>
>> We also need to fix the Appendix to describe about control transport and full transport.
>> Current definition of Appendix C is listing MMIO And PCI transport, yet it misses virtqueue implementation by _full_ transport.
>
> because they all share a single virtqueue implementation.
>
>>>
>>>
>>>
>>>>
>>>> And hence, data transfer part must be scoped properly.
>>>> Maybe I am yet to read this text in the 1600 lines of proposed patch...
>>>
>>>
>>> Once we get into "support everything in the most abstract way possible"
>>> we have already lost.
>> Unlikely.
>> Nvme over TCP is present since 2021.
>> NVMe over RDMA is present since 2017.
>> iSCSI over TCP is present from 2014.
>> NFS ..
>> List continues...
>>
>> The idea is to not define most abstract thing.
>> The idea is to define the practical spec that cater to requirements already listed, including the TCP one in [1].
>
> I don't object to virtio over TCP. just do not want to block this
> work on that.
>
>
>>> No one asked for virtio over carrier pigeons.
>> CSP user already explained the use case of virtio over network transport in [1].
>> Considering above many use cases already done on similar non virtio devices as pigeons is severe undermining the scope of virtio.
>>
>> One can say, I only want to engineer control transport ignoring needs of [1].
>> However, we must have the doors and vision open so that users of [1] can also use it without creating yet another 'fabric transport'.
>>
>> At the same time, we must define the binding to the bus where it is going to be used for the DMA.
>>
>> For example, cover letter mentions about " between a host processor and its co-processors"
>> And " between normal and secure worlds"
>>
>> How can one implement a driver if the bus driver does not know how to enumerate/discover it?
>>
>> In the proposed example, of host processor and co-processor if they are connected via Xenbus or AMBA bus,
>> how should driver discover this device?
>>
>> If this bus binding is not part of the virtio spec, how can it be ever implemented and yet comply to the virtio spec?
>> Can someone please explain this?
>>
>> I frankly expect an ARM FF-A bus binding transport section in virtio-spec.
>> So that device and driver can inter-operate implemented by two different entities.
>> Only virtio level messages do not seem sufficient.
>
> I agree at least one should be shown at least as an rfc.

Here is a link to the current virtio message bus over FF-A specification:
https://developer.arm.com/documentation/den0153/0101/

1.0 Alpha 1 version in sync with this version of the virtio message spec will be released later today, you will be redirected to
1.0 Alpha 0 version until it is which is in sync with the previous RFC we sent.

Regards
Bertrand

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