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

Manivannan Sadhasivam
2026-02-04T05:34:27Z

On Mon, Jan 26, 2026 at 11:32:26AM -0500, Bill Mills wrote:
> 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.
>
> We are working on a few reusable bus implementations:
>
> * virtio-msg-ffa based on Arm FF-A interface for use between:
> * normal world and secure world
> * host and VM or VM to VM
> * Can be used w/ or with out a hypervisor
> * Any Hypervisor that implements FF-A can be used
> * We have this working with pKVM and Xen
> * We have this working with Trusty and OP-TEE
>
> * virtio-msg-amp for use between heterogenous systems
> * The main processors and its co-processors on an AMP SOC
> * Two or more systems connected via PCIe
> * Minimal requirements: bi-directional interrupts and
> at least one shared memory area
> * hvac-demo has 2 demos of this
> * This is working on two hardware platforms
>
> * virtio-msg-loopback for userspace implemented devices
> * Allows user space to provide devices to its own kernel
> * This is similar to fuse, cuse or loopback block devices but for virtio
> * hvac-demo has a demo
>
> We also anticipate a few more:
>
> * virtio-msg-xen specific to Xen
> * Usable on any Xen system (including x86 where FF-A does not exist)
> * Using Xen events and page grants
>
> * virtio-msg over admin virtqueues
> * This allows any virtio-pci device that supports admin virtqueues to also
> support a virtio-msg bus that supports sub devices
> * [We are looking for collaborators for this work]
>
> Changes since RFC2:
>
> Spec Functional:
> * Made the common message header 8 bytes and added a token for optional use
> when parallel outstanding requests are possible
> * Made the 8 byte fields align to 8 byte offsets.
> This effects the {SET,GET}_VQUEUE messages
> * Many conformance cases have been tightened.
>
> Spec Editorial:
> * Major restructure to better align with virtio spec
> * Conformance model now matches other transports
> * Use C structures to define messages instead of tables
> * Added a section to describe how responses are matched to requests
> This includes the use of the new token field
> * Redefined / better described error handling between transport and bus
> layers to eliminate the need for the bus to generate fake response messages
> * Included editorial feedback from RFC2
>
> Eco-system:
> * Added virtio-msg-loopback demo
> * Arm has published the first draft of the virtio-msg over FFA spec [6]
> * virtio-msg over FFA has been demonstrated with both Trusty and OP-TEE
> secure world
> * LKML RFC has been sent [7]
> * QEMU RFC has been sent [8]
>
> This is the first non-RFC patch series. The known short comings have been
> addressed. We ask for review in earnest on this series and thank you for
> any feedback you can provide.
>
> Background info and work in progress implementations:
> * HVAC project page with intro slides [1]
> * HVAC demo repo w/ instructions in README.md [2]
> * Kernel w/ virtio-msg common level and ffa support [3]
> * QEMU w/ support for one form of virtio-msg-amp [4]
> * Portable RTOS library w/ one form of virtio-msg-amp [5]
>
> In addition to the QEMU system based demos in the hvac-demo repo, we also have
> two hardware systems running:
> * AMD x86 + AMD Arm Versal connected via PCIe
> * ST STM32MP157 A7 Linux using virtio-i2c provided by M4 Zephyr
>
> Please note that although the demos work, a few have not been aligned
> with this version of the spec.
>

FWIW,

Acked-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>

- Mani

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