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

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

On Wed, Feb 25, 2026 at 03:36:36PM +0530, Manivannan Sadhasivam wrote:
> On Wed, Feb 25, 2026 at 08:03:48AM +0000, Bertrand Marquis wrote:
> > Hi Manivannan,
> > 
> > > On 25 Feb 2026, at 08:45, Manivannan Sadhasivam <mani@kernel.org> wrote:
> > >
> > > Hi Bertrand,
> > >
> > > On Fri, Feb 20, 2026 at 09:02:12AM +0000, Bertrand Marquis wrote:
> > >> Hi Parav,
> > >>
> > >>> On 20 Feb 2026, at 07:13, Parav Pandit <parav@nvidia.com> wrote:
> > >>>
> > >>>
> > >>>
> > >>>> From: Michael S. Tsirkin <mst@redhat.com>
> > >>>> Sent: 20 February 2026 05:25 AM
> > >>>>
> > >>>> On Fri, Feb 13, 2026 at 01:52:06PM +0000, Parav Pandit wrote:
> > >>>>> Hi Bill,
> > >>>>>
> > >>>>>> From: Bill Mills <bill.mills@linaro.org>
> > >>>>>> Sent: 26 January 2026 10:02 PM
> > >>>>>>
> > >>>>>> This series adds the virtio-msg transport layer.
> > >>>>>>
> > >>>>>> The individuals and organizations involved in this effort have had difficulty in
> > >>>>>> using the existing virtio-transports in various situations and desire to add one
> > >>>>>> more transport that performs its transport layer operations by sending and
> > >>>>>> receiving messages.
> > >>>>>>
> > >>>>>> Implementations of virtio-msg will normally be done in multiple layers:
> > >>>>>> * common / device level
> > >>>>>> * bus level
> > >>>>>>
> > >>>>>> The common / device level defines the messages exchanged between the driver
> > >>>>>> and a device. This common part should lead to a common driver holding most
> > >>>>>> of the virtio specifics and can be shared by all virtio-msg bus implementations.
> > >>>>>> The kernel implementation in [3] shows this separation. As with other transport
> > >>>>>> layers, virtio-msg should not require modifications to existing virtio device
> > >>>>>> implementations (virtio-net, virtio-blk etc). The common / device level is the
> > >>>>>> main focus of this version of the patch series.
> > >>>>>>
> > >>>>>> The virtio-msg bus level implements the normal things a bus defines
> > >>>>>> (enumeration, dma operations, etc) but also implements the message send and
> > >>>>>> receive operations. A number of bus implementations are envisioned,
> > >>>>>> some of which will be reusable and general purpose. Other bus implementations
> > >>>>>> might be unique to a given situation, for example only used by a PCIe card
> > >>>>>> and its driver.
> > >>>>>>
> > >>>>>> The standard bus messages are an effort to avoid different bus implementations
> > >>>>>> doing the same thing in different ways for no good reason. However the
> > >>>>>> different environments will require different things. Instead of trying to
> > >>>>>> anticipate all needs and provide something very abstract, we think
> > >>>>>> implementation specific messages will be needed at the bus level. Over time,
> > >>>>>> if we see similar messages across multiple bus implementations, we will move to
> > >>>>>> standardize a bus level message for that.
> > >>>>>>
> > >>>>>
> > >>>>> I would review more, had first round of sparse review.
> > >>>>> Please find few comments/questions below.
> > >>>>
> > >>>> I'd like to comment that I think it makes sense to have a basic simple transport and
> > >>>> then add performance features on top as appropriate.
> > >>> Sounds good. Simple but complete is needed.
> > >>
> > >> Agree.
> > >>
> > >>>
> > >>>> So one way to address some of these comments is to show how
> > >>>> they can be addressed with a feature bit down the road.
> > >>>>
> > >>>>
> > >>>>> 1. device number should be 32-bit in struct virtio_msg_header.
> > >>>>>> From SIOV_R2 experiences, we learnt that some uses have use case for more than 64k devices.
> > >>>>> Also mapping PCI BDF wont be enough in 16-bits considering domain field.
> > >>>>>
> > >>>>> 2. msg_size of 16-bits for 64KB-8 bytes is too less for data transfer.
> > >>>>> For example, a TCP stream wants to send 64KB of data + payload, needs more than 64KB data.
> > >>>>> Needs 32-bits.
> > >>>>>
> > >>>>> 3. BUS_MSG_EVENT_DEVICE to have symmetric name as ADDED and REMOVED (instead of READY)
> > >>>>> But more below.
> > >>>>>
> > >>>>> 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?
> > >>
> > >> As said in a previous mail, we have messages already for that.
> > >> Please confirm if that answer your question.
> > >>
> > >>> How can one implement a transport without defining the basic data transfer semantics?
> > >>
> > >> We did a lot of experiments and we are feature equivalent to PCI, MMIO or Channel I/O.
> > >> If anything is missing, we are more than happy to discuss it and solve the issue.
> > >>
> > >
> > > I'd love to have this transport over PCI because it addresses the shortcomings
> > > of the existing PCI transport which just assumes that every config space access\
> > > is trap and emulate.
> > 
> > Agree and AMD did exactly that in their demonstrator.
> > I will give you answers here as i know them but Edgar will probably give you more
> > details (and probably fix my mistakes).
> > 
> > >
> > > But that being said, I somewhat agree with Parav that we should define the bus
> > > implementations in the spec to avoid fixing the ABI in the implementations. For
> > > instance, if we try to use this transport over PCI, we've got questions like:
> > >
> > > 1. How the device should be bind to the virtio-msg-pci bus driver and not with
> > > the existing virtio-pci driver? Should it use a new Vendor ID or Sub-IDs?
> > 
> > One bus is appearing as one pci device with its own Vendor ID,
> > 
> 
> What should be the 'own Vendor ID' here?
> 
> The existing virtio-pci driver binds to all devices with the Vendor ID of
> PCI_VENDOR_ID_REDHAT_QUMRANET. So are you expecting the Vendors to use their own
> VID for exposing the Virtio devices? That would mean, the drivers on the host
> need update as well, which will not scale.
> 
> It would be good if the existing virtio-pci devices can use this new transport
> with only device side modifications.

Oh another transitional device hack(

You guys should think hard whether you really want that.


> > >
> > > 2. How the Virtio messages should be transferred? Is it through endpoint config
> > > space or through some other means?
> > 
> > The virtio messages are transfered using FIFOs stored in the BAR of the PCI
> > device (ending up being memory shared between both sides)
> > 
> 
> What should be the BAR number and size?
> 
> > >
> > > 3. How the notification be delivered from the device to the host? Through
> > > INT-X/MSI/MSI-X or even polling?
> > 
> > Notifications are delivered through MSI.
> > 
> 
> So no INT-X or MSI-X? Why so?
> 
> Anyhow, my objective is not to get answers for my above questions here in this
> thread, but to state the reality that it would be hard for us to make use of
> this new transport without defining the bus implementation.
> 
> > >
> > > And these are just a few questions that comes to the top of my head. There could
> > > be plenty more.
> > >
> > > How can we expect all the virtio-msg bus implementations to adhere to the same
> > > format so that the interoperability offered by the Virtio spec is guaranteed?
> > 
> > We spent a lot of time thinking on that (this started around 2 years ago) and we
> > discussed several use cases and did some PoC to try to have everything covered
> > (secure to non secure and vm to vm using ffa, system to system over PCI or hardware
> > messaging system, PCI, Xen specific implementation) to check the needs and try to
> > cover as much as we can.
> > 
> > Now there might be cases we missed but we think that having a purely message based
> > interface between the bus and the transport and split responsibilities the way we did
> > is allowing lots of different bus implementations without affecting the transport and
> > driver/device implementations on top.
> > 
> > We identified that a common use case will be for the bus to transfer messages using
> > FIFOs to optimize speed (at the end you need to have a way to share memory between
> > both sides so why not using a part of it to transfer the messages to and reduce the number
> > of data exchanges and copies) and this will be used by PCI, Xen, FF-A and others in
> > practice (so we might standardize the FIFO format in the future to allow even more code
> > reuse between busses).
> > 
> 
> Not just the FIFO format, but how that FIFO gets shared between the device and
> the host also needs to be documented. Maybe for this initial transport version,
> you can start with defining the FF-A bus implementation?
> 
> - Mani
> 
> -- 
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