Re: [RFC PATCH v3 4/5] rust: miscdevice: Add `uring_cmd` support

[Caleb Sander Mateos <csander@purestorage.com>]

On Fri, Aug 22, 2025 at 5:56 AM Sidong Yang <sidong.yang@furiosa.ai> wrote:
>
> This patch introduces support for `uring_cmd` to the `miscdevice`
> framework. This is achieved by adding a new `uring_cmd` method to the
> `MiscDevice` trait and wiring it up to the corresponding
> `file_operations` entry.
>
> The `uring_cmd` function provides a mechanism for `io_uring` to issue
> commands to a device driver.
>
> The new `uring_cmd` method takes the device, an `IoUringCmd` object,
> and issue flags as arguments. The `IoUringCmd` object is a safe Rust
> abstraction around the raw `io_uring_cmd` struct.
>
> To enable `uring_cmd` for a specific misc device, the `HAS_URING_CMD`
> constant must be set to `true` in the `MiscDevice` implementation.
>
> Signed-off-by: Sidong Yang <sidong.yang@furiosa.ai>
> ---
>  rust/kernel/miscdevice.rs | 53 ++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 52 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
> index 6373fe183b27..fcef579218ba 100644
> --- a/rust/kernel/miscdevice.rs
> +++ b/rust/kernel/miscdevice.rs
> @@ -11,9 +11,10 @@
>  use crate::{
>      bindings,
>      device::Device,
> -    error::{to_result, Error, Result, VTABLE_DEFAULT_ERROR},
> +    error::{from_result, to_result, Error, Result, VTABLE_DEFAULT_ERROR},
>      ffi::{c_int, c_long, c_uint, c_ulong},
>      fs::File,
> +    io_uring::IoUringCmd,
>      mm::virt::VmaNew,
>      prelude::*,
>      seq_file::SeqFile,
> @@ -180,6 +181,21 @@ fn show_fdinfo(
>      ) {
>          build_error!(VTABLE_DEFAULT_ERROR)
>      }
> +
> +    /// Handler for uring_cmd.
> +    ///
> +    /// This function is invoked when userspace process submits an uring_cmd op
> +    /// on io-uring submission queue. The `device` is borrowed instance defined
> +    /// by `Ptr`. The `io_uring_cmd` would be used for get arguments cmd_op, sqe,
> +    /// cmd_data. The `issue_flags` is the flags includes options for uring_cmd.
> +    /// The options are listed in `kernel::io_uring::cmd_flags`.
> +    fn uring_cmd(
> +        _device: <Self::Ptr as ForeignOwnable>::Borrowed<'_>,
> +        _io_uring_cmd: Pin<&mut IoUringCmd>,

Passing the IoUringCmd by reference doesn't allow the uring_cmd()
implementation to store it beyond the function return. That precludes
any asynchronous uring_cmd() implementation, which is kind of the
whole point of uring_cmd. I think uring_cmd() needs to transfer
ownership of the IoUringCmd so the implementation can complete it
asynchronously.

Best,
Caleb

> +        _issue_flags: u32,
> +    ) -> Result<i32> {
> +        build_error!(VTABLE_DEFAULT_ERROR)
> +    }
>  }
>
>  /// A vtable for the file operations of a Rust miscdevice.
> @@ -337,6 +353,36 @@ impl<T: MiscDevice> MiscdeviceVTable<T> {
>          T::show_fdinfo(device, m, file);
>      }
>
> +    /// # Safety
> +    ///
> +    /// The caller must ensure that:
> +    /// - The pointer `ioucmd` is not null and points to a valid `bindings::io_uring_cmd`.
> +    unsafe extern "C" fn uring_cmd(
> +        ioucmd: *mut bindings::io_uring_cmd,
> +        issue_flags: ffi::c_uint,
> +    ) -> c_int {
> +        // SAFETY: `file` referenced by `ioucmd` is valid pointer. It's assigned in
> +        // uring cmd preparation. So dereferencing is safe.
> +        let raw_file = unsafe { (*ioucmd).file };
> +
> +        // SAFETY: `private_data` is guaranteed that it has valid pointer after
> +        // this file opened. So dereferencing is safe.
> +        let private = unsafe { (*raw_file).private_data }.cast();
> +
> +        // SAFETY: `ioucmd` is not null and points to valid memory `bindings::io_uring_cmd`
> +        // and the memory pointed by `ioucmd` is valid and will not be moved or
> +        // freed for the lifetime of returned value `ioucmd`
> +        let ioucmd = unsafe { IoUringCmd::from_raw(ioucmd) };
> +
> +        // SAFETY: This call is safe because `private` is returned by
> +        // `into_foreign` in [`open`]. And it's guaranteed
> +        // that `from_foreign` is called by [`release`] after the end of
> +        // the lifetime of `device`
> +        let device = unsafe { <T::Ptr as ForeignOwnable>::borrow(private) };
> +
> +        from_result(|| T::uring_cmd(device, ioucmd, issue_flags))
> +    }
> +
>      const VTABLE: bindings::file_operations = bindings::file_operations {
>          open: Some(Self::open),
>          release: Some(Self::release),
> @@ -359,6 +405,11 @@ impl<T: MiscDevice> MiscdeviceVTable<T> {
>          } else {
>              None
>          },
> +        uring_cmd: if T::HAS_URING_CMD {
> +            Some(Self::uring_cmd)
> +        } else {
> +            None
> +        },
>          // SAFETY: All zeros is a valid value for `bindings::file_operations`.
>          ..unsafe { MaybeUninit::zeroed().assume_init() }
>      };
> --
> 2.43.0
>