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

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

On Wed, Feb 25, 2026 at 10:35:41AM +0000, Bertrand Marquis wrote:
> Hi Parav,
> 
> > On 25 Feb 2026, at 11:24, Parav Pandit <parav@nvidia.com> wrote:
> >
> >>
> >> From: Manivannan Sadhasivam <mani@kernel.org>
> >> Sent: 25 February 2026 03:37 PM
> >>
> >> 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.
> >>
> >>>>
> >>>> 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.
> >>
> > +1 to most of the points that Manivannan explained.
> >
> > The whole new definition of message layer for the PCI does not make any sense at all where expectation for the device is to build yet another interface for _Everything_ that already exists.
> > and device is still have to implement all the existing things because the device does not know which driver will operate.
> >
> > And that too some register based inefficient interface.
> > Just to reset the device one needs to fully setup the new message interface but device still have to be working.
> > That defeats the whole purpose of reset_1 and reset_2 in the device.
> >
> > This does not bring anything better for the PCI devices at all.
> >
> > A transport binding should be defined for the bus binding.
> > A bus that chooses a msg interface should be listed that way and bus choose inline messages can continue the way they are.
> >
> > If we are creating something brand-new, for PCI the only thing needed is:
> > 1. Reset the device
> > 2. Create an admin virtqueue
> > 3. Transport everything needed through this virtqueue including features, configs, control.
> >
> > And this will work for any other bus or msg based too given only contract needed is to creating the aq.
> 
> I think you misunderstood a bit the point of virtio-msg bus over PCI so let me try to explain.
> 
> You see one PCI device (regular, not virtio) which is a "virtio-msg bus over PCI".
> 
> When the virtio-msg bus over PCI it will communicate through this device with an external
> system connected through the PCI bus.
> The driver will enumerate virtio devices available behind this bus and register them so that
> the corresponding virtio drivers are probed for them.
> All virtio-msg messages required to communicate with those devices will be transferred through
> a FIFO stored in the BAR of the pci device and standard PCI DMA will be used to share the
> virtqueues with all the devices on the bus.
> 
> So the PCI device is not one virtio device but one bus behind which there can be many devices.
> 
> Is this making the concept a bit clearer ?
> 
> Cheers
> Bertrand
> 

So this is fundamentally pci over virtio-msg?


> >
> >>>>
> >>>> 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?
> 
>