Re: [RFC 0/2] rust: introduce abstractions for fwctl

Zhi Wang
2025-10-30T17:29:10Z

On 30.10.2025 18.03, Zhi Wang wrote:
> In the NVIDIA vGPU RFC [1], the vGPU type blobs must be provided to the GSP
> before userspace can enumerate available vGPU types and create vGPU
> instances. The original design relied on the firmware loading interface,
> but fwctl is a more natural fit for this use case, as it is designed for
> uploading configuration or firmware data required before the device becomes
> operational.
>

Hi Jason and Danilo:

Thanks for the comments. I had one more open to discuss, handling the buffer
allocation/free between rust and C world.

Two fwctl ioctls:

FWCTL_CMD_INFO: The driver allocates the info memory (kmalloc) and the fwctl
subsystem frees it.

FWCTL_RPC:

Case 1: The driver can choose to re-use the input buffer and write the *out_len
for actual length of data.

Case 2: The driver can allocate a new buffer (kmalloc) and the fwctl subsystem
frees it.

----
Now with the Rust driver:

FWCTL_CMD_INFO: The rust side returns a new KVec, the rust fwctl abstraction
consumes it, get void *buf and pass it to fwctl subsystem (C). The memory
will be freed by C side.

FWCTL_RPC:

The input buffer will be wrapped in a mutable slice.

Case 1: Re-use the input buffer. The rust side writes the mut slice and the
* mut out_len.

Case 2: Allocate the new output buffer. The same approach as FWCTL_CMD_INFO.

----

We know KVec is backed by kmalloc. If C side changes the requirements of
the driver memory allocation someday, E.g. from kfree() to kvfree() or vfree().

Drivers in C will be updated surely at that time.

Is possible that we can have some approaches to catch that change from the rust
side via rust compiler for rust drivers?

Z.

> This patch introduces a Rust abstraction over the fwctl subsystem,
> providing safe and idiomatic bindings.
>
> The new `fwctl` module allows Rust drivers to integrate with the existing
> C-side fwctl core through a typed trait interface. It provides:
>
> - `FwCtlOps` trait — defines driver-specific operations such as
> `open_uctx()`, `close_uctx()`, `info()`, and `fw_rpc()`.
> Each Rust driver implements this trait to describe its own per-FD
> user-context behavior and RPC handling.
>
> - `FwCtlUCtx<T>` — a generic wrapper around `struct fwctl_uctx`
> embedding driver-specific context data, providing safe conversion
> from raw C pointers and access to the parent device.
>
> - `Registration<T>` — safe registration and automatic unregistration
> of `struct fwctl_device` objects using the kernel’s device model.
>
> - `FwCtlVTable<T>` — a static vtable bridging C callbacks and Rust
> trait methods, ensuring type safety across the FFI boundary.
>
> `rust/kernel/lib.rs` is updated to conditionally include this module
> under `CONFIG_FWCTL`.
>
> [1] https://lore.kernel.org/all/20250903221111.3866249-1-zhiw@nvidia.com/
>
> Zhi Wang (2):
> rust: introduce abstractions for fwctl
> samples: rust: fwctl: add sample code for FwCtl
>
> include/uapi/fwctl/fwctl.h | 1 +
> rust/bindings/bindings_helper.h | 1 +
> rust/kernel/fwctl.rs | 254 ++++++++++++++++++++++++++++++
> rust/kernel/lib.rs | 2 +
> samples/rust/Kconfig | 11 ++
> samples/rust/Makefile | 1 +
> samples/rust/rust_driver_fwctl.rs | 123 +++++++++++++++
> 7 files changed, 393 insertions(+)
> create mode 100644 rust/kernel/fwctl.rs
> create mode 100644 samples/rust/rust_driver_fwctl.rs
>