[PATCH RFC v2 0/6] Add Rust virtio bindings and sample device

[Manos Pitsidianakis <manos@pitsidianak.is>]

Hi all, this RFC series adds Rust bindings for Virtio drivers
(frontends in virtio parlance).

As a PoC, it also adds a sample virtio-rtc driver which performs
capability discovery through the virtqueue without registering any clock.

Before I send a cleaned-up non-RFC I would like some initial feedback
(i.e. is it something the upstream wants?)

This was tested with the rust-vmm vhost-device-rtc device backend that I
wrote[^0]:

[^0]: https://github.com/rust-vmm/vhost-device/tree/main/vhost-device-rtc

Instructions:

  Run the daemon in a separate terminal:

  $ cargo run --bin vhost-device-rtc -- -s /tmp/rtc.sock

  Then run the VM:

  $ qemu-system-aarch64 \
    -machine type=virt,virtualization=off,acpi=on \
    -cpu host \
    -smp 8 \
    -accel kvm \
    -drive if=virtio,format=qcow2,file=./debian-13-nocloud-arm64-daily.qcow2 \
    -device virtio-net-pci,netdev=unet \
    -device virtio-scsi-pci \
    -serial mon:stdio \
    -m 8192 \
    -object memory-backend-memfd,id=mem,size=8G,share=on \
    -numa node,memdev=mem \
    -display none \
    -vga none \
    -kernel /path/to/linux/build/arch/arm64/boot/Image \
    -device vhost-user-test-device,chardev=rtc,id=rtc,virtio-id=17,num_vqs=2,vq_size=1024 \
    -chardev socket,path=/tmp/rtc.sock,id=rtc \
    ...

  Example output:
    [    1.105238] rust_virtio_rtc: Probe Rust virtio driver sample.
    [    1.105645] rust_virtio_rtc: Found 1 vqs.
    [    1.136050] rust_virtio_rtc: process_requestq got buf 16 bytes
    [    1.136125] rust_virtio_rtc: Got response! Ok(RespCfg { head: ReqHead { msg_type: Le16(0), reserved: [0, 0, 0, 0, 0, 0] }, num_clocks: Le16(3), reserved: [0, 0, 0, 0, 0, 0] })
    [    1.136701] rust_virtio_rtc: Got response! Ok(RespClockCap { head: ReqHead { msg_type: Le16(0), reserved: [0, 0, 0, 0, 0, 0] }, clock_type: 3, leap_second_smearing: 0, flags: 0, reserved: [0, 0, 0, 0, 0] })
    [    1.136724] rust_virtio_rtc virtio0: cannot expose clock 0 (type 3, variant 0, flags 0) to userspace
    [    1.137259] rust_virtio_rtc: Got response! Ok(RespRead { head: ReqHead { msg_type: Le16(0), reserved: [0, 0, 0, 0, 0, 0] }, clock_reading: Le64(1777890485031060388) })
    [    1.137277] rust_virtio_rtc: #0 clock reading = 1777890485031060388
    [    1.137749] rust_virtio_rtc: Got response! Ok(RespClockCap { head: ReqHead { msg_type: Le16(0), reserved: [0, 0, 0, 0, 0, 0] }, clock_type: 1, leap_second_smearing: 0, flags: 0, reserved: [0, 0, 0, 0, 0] })
    [    1.137769] rust_virtio_rtc virtio0: cannot expose clock 1 (type 1, variant 0, flags 0) to userspace
    [    1.138247] rust_virtio_rtc: Got response! Ok(RespRead { head: ReqHead { msg_type: Le16(0), reserved: [0, 0, 0, 0, 0, 0] }, clock_reading: Le64(1777890485032086075) })
    [    1.138264] rust_virtio_rtc: #1 clock reading = 1777890485032086075
    [    1.138730] rust_virtio_rtc: Got response! Ok(RespClockCap { head: ReqHead { msg_type: Le16(0), reserved: [0, 0, 0, 0, 0, 0] }, clock_type: 2, leap_second_smearing: 0, flags: 0, reserved: [0, 0, 0, 0, 0] })
    [    1.138751] rust_virtio_rtc virtio0: cannot expose clock 2 (type 2, variant 0, flags 0) to userspace
    [    1.139253] rust_virtio_rtc: Got response! Ok(RespRead { head: ReqHead { msg_type: Le16(0), reserved: [0, 0, 0, 0, 0, 0] }, clock_reading: Le64(338567896865557) })
    [    1.139270] rust_virtio_rtc: #2 clock reading = 338567896865557

Concerns - Notes - TODOs
========================

- Virtqueue lifetimes don't neatly apply to Rust as expected, so a lot
  of times we have to go through unsafe pointer dereferences (though
  which are guaranteed by Virtio subsystem to be valid, for example when
  a callback is called with the vq argument). There's a potential for
  misuse and definitely could use better thinking.
- `struct virtio_device` is not reference-counted like other implemented
  device types in rust/kernel. Maybe we need to change C API first to
  make them reference counted, assuming this doesn't break anything?
- Not sure if adding data smaller than PAGE_SIZE to virtqueue is safe
  (which current C code does a lot), so I made those allocations at
  least PAGE_SIZE.
- The sample driver obviously conflicts with the C implementation, so
  this would either need to move out of samples/ or figure out some way
  to handle this in kbuild.
- kernel::virtio module and its types need a few rustdoc examples that I
  will add in followup series
- Note that the registration of RTC clocks etc in the sample driver is
  not done, I'm putting it off until I receive some feedback first. The
  sample driver otherwise does send and receive data from the virtqueue
  as a PoC. The code and data structures can be cleaned up further
  before followup series.

PS: No LLMs used so any mistakes and goofs are solely written by me.

Signed-off-by: Manos Pitsidianakis <manos@pitsidianak.is>
---
Changes in v2:
- Move helper ifdefs to helper file (thanks Alice)
- Changed CONFIG checks to IS_ENABLED to allow for CONFIG_VIRTIO=m
- Split all use imports to one item per line according to style guide
- Fixed wait_for_completion_interruptible*() rustdocs
- Use Jiffy type alias in wait_for_completion_interruptible_timeout()
- Pepper and salt #[inline]s wherever appropriate as per style guide
- Split probe() into probe() and init() to allow cleaning up if init
  fails
- Remove unnecessary Send and Sync unsafe impls for
  kernel::virtio::Device
- Remove unnecessary LeSize and BeSize 
- Accept Option<_> for virtqueue callback when creating a VirtqueueInfo
- Made all vq buffer adding operations unsafe
- Use AtomicU16 instead of Cell<u16> for sample virtio driver
- Fix RespHead field types in sample virtio driver
- Fix response error checking in sample virtio driver
- Change some device contexts in method signatures
- Link to v1: https://lore.kernel.org/r/20260505-rust-virtio-v1-0-9563383909e4@pitsidianak.is

---
Manos Pitsidianakis (6):
      rust/bindings: generate virtio bindings
      rust/helpers: add virtio.c
      rust: add virtio module
      rust/scatterlist: add SGEntry::init_one
      rust: impl interruptible waits for Completion
      samples/rust: Add sample virtio-rtc driver [WIP]

 MAINTAINERS                     |   9 +
 rust/bindings/bindings_helper.h |   5 +
 rust/helpers/helpers.c          |   1 +
 rust/helpers/virtio.c           |  37 +++
 rust/kernel/lib.rs              |   2 +
 rust/kernel/scatterlist.rs      |  20 +-
 rust/kernel/sync/completion.rs  |  42 +++-
 rust/kernel/virtio.rs           | 419 ++++++++++++++++++++++++++++++++++
 rust/kernel/virtio/utils.rs     |  63 ++++++
 rust/kernel/virtio/virtqueue.rs | 241 ++++++++++++++++++++
 samples/rust/Kconfig            |  15 ++
 samples/rust/Makefile           |   1 +
 samples/rust/rust_virtio_rtc.rs | 491 ++++++++++++++++++++++++++++++++++++++++
 13 files changed, 1344 insertions(+), 2 deletions(-)
---
base-commit: 028ef9c96e96197026887c0f092424679298aae8
change-id: 20260504-rust-virtio-8523b01dfdc2

Best regards,
-- 
Manos Pitsidianakis <manos@pitsidianak.is>