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

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Message-ID: <8e0209de-5be3-4d41-807f-1b7e73aa3d1d@nvidia.com>
Date: Thu, 30 Oct 2025 17:29:06 +0000
From: Zhi Wang <zhiw@...dia.com>
To: "rust-for-linux@...r.kernel.org" <rust-for-linux@...r.kernel.org>
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 Fernandes <joelagnelf@...dia.com>, John Hubbard <jhubbard@...dia.com>, Jason
 Gunthorpe <jgg@...dia.com>
Subject: Re: [RFC 0/2] rust: introduce abstractions for fwctl

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
>