Re: [RFC PATCH v2 4/4] samples: rust: rust_misc_device: add uring_cmd example

[Daniel Almeida <daniel.almeida@collabora.com>]

Hi Sidong,

> On 27 Jul 2025, at 12:03, Sidong Yang <sidong.yang@furiosa.ai> wrote:
> 
> This patch makes rust_misc_device handle uring_cmd. Command ops are like
> ioctl that set or get values in simple way.
> 
> Signed-off-by: Sidong Yang <sidong.yang@furiosa.ai>
> ---
> samples/rust/rust_misc_device.rs | 34 ++++++++++++++++++++++++++++++++
> 1 file changed, 34 insertions(+)
> 
> diff --git a/samples/rust/rust_misc_device.rs b/samples/rust/rust_misc_device.rs
> index c881fd6dbd08..1044bde86e8d 100644
> --- a/samples/rust/rust_misc_device.rs
> +++ b/samples/rust/rust_misc_device.rs
> @@ -101,6 +101,7 @@
>     c_str,
>     device::Device,
>     fs::File,
> +    io_uring::IoUringCmd,
>     ioctl::{_IO, _IOC_SIZE, _IOR, _IOW},
>     miscdevice::{MiscDevice, MiscDeviceOptions, MiscDeviceRegistration},
>     new_mutex,
> @@ -114,6 +115,9 @@
> const RUST_MISC_DEV_GET_VALUE: u32 = _IOR::<i32>('|' as u32, 0x81);
> const RUST_MISC_DEV_SET_VALUE: u32 = _IOW::<i32>('|' as u32, 0x82);
> 
> +const RUST_MISC_DEV_URING_CMD_SET_VALUE: u32 = _IOR::<i32>('|' as u32, 0x83);
> +const RUST_MISC_DEV_URING_CMD_GET_VALUE: u32 = _IOW::<i32>('|' as u32, 0x84);
> +
> module! {
>     type: RustMiscDeviceModule,
>     name: "rust_misc_device",
> @@ -190,6 +194,36 @@ fn ioctl(me: Pin<&RustMiscDevice>, _file: &File, cmd: u32, arg: usize) -> Result
> 
>         Ok(0)
>     }
> +
> +    fn uring_cmd(
> +        me: Pin<&RustMiscDevice>,

“me” ?

> +        io_uring_cmd: Pin<&mut IoUringCmd>,
> +        _issue_flags: u32,
> +    ) -> Result<i32> {
> +        dev_info!(me.dev, "UringCmd Rust Misc Device Sample\n");
> +
> +        let cmd = io_uring_cmd.cmd_op();
> +        let cmd_data = io_uring_cmd.sqe().cmd_data().as_ptr() as *const usize;
> +
> +        // SAFETY: `cmd_data` is guaranteed to be a valid pointer to the command data
> +        // within the SQE structure.

This is what core::ptr::read_volatile says:

Safety
Behavior is undefined if any of the following conditions are violated:
    • src must be valid for reads.
    • src must be properly aligned.
    • src must point to a properly initialized value of type T.

You must prove that the pre-conditions above are fulfilled here.

> +        // FIXME: switch to read_once() when it's available.
> +        let addr = unsafe { core::ptr::read_volatile(cmd_data) };

So drivers have to write “unsafe” directly? It isn’t forbidden, but
we should try our best to avoid it.

> +
> +        match cmd {
> +            RUST_MISC_DEV_URING_CMD_SET_VALUE => {
> +                me.set_value(UserSlice::new(addr, 8).reader())?;
> +            }
> +            RUST_MISC_DEV_URING_CMD_GET_VALUE => {
> +                me.get_value(UserSlice::new(addr, 8).writer())?;
> +            }
> +            _ => {
> +                dev_err!(me.dev, "-> uring_cmd not recognised: {}\n", cmd);
> +                return Err(ENOTTY);
> +            }
> +        }
> +        Ok(0)
> +    }
> }
> 

Who calls this function?

> #[pinned_drop]
> -- 
> 2.43.0
> 
> 

— Daniel