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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Feb 25, 2026 at 09:35:29AM +0000, Bertrand Marquis wrote:
> 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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thanks. pls include that in the series as [PATCH RFC 5/4]