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[80.230.79.166]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-483bd6f26d7sm68507005e9.3.2026.02.25.02.52.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 25 Feb 2026 02:52:14 -0800 (PST) Date: Wed, 25 Feb 2026 05:52:11 -0500 From: "Michael S. Tsirkin" To: Bertrand Marquis Cc: Parav Pandit , Manivannan Sadhasivam , "Bill Mills (bill.mills@linaro.org)" , "virtio-comment@lists.linux.dev" , "Edgar E . Iglesias" , Arnaud Pouliquen , Viresh Kumar , Alex Bennee , Armelle Laine Subject: Re: [PATCH v1 0/4] virtio-msg transport layer Message-ID: <20260225055020-mutt-send-email-mst@kernel.org> References: <20260126163230.1122685-1-bill.mills@linaro.org> <20260219185034-mutt-send-email-mst@kernel.org> <359B0C17-9D57-423A-A229-6CEDA19C975A@arm.com> <02226901-7670-4AAB-8F55-0B2FB7C0CA49@arm.com> Precedence: bulk X-Mailing-List: virtio-comment@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <02226901-7670-4AAB-8F55-0B2FB7C0CA49@arm.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: q0GpUfULZiJG_XpMIZdqnvwjAjpY6FYo8A4oxAW7LM0_1772016736 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Wed, Feb 25, 2026 at 10:35:41AM +0000, Bertrand Marquis wrote: > Hi Parav, > > > On 25 Feb 2026, at 11:24, Parav Pandit wrote: > > > >> > >> From: Manivannan Sadhasivam > >> 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 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 wrote: > >>>>>> > >>>>>> > >>>>>> > >>>>>>> From: Michael S. Tsirkin > >>>>>>> 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 > >>>>>>>>> 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? > >