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Message-ID: <CADUfDZoDvAp1yqFyB_SQiynqQfOQPkO_mnQ_pWAXpZJESecFFw@mail.gmail.com>
Date: Mon, 1 Sep 2025 18:12:44 -0700
From: Caleb Sander Mateos <csander@...estorage.com>
To: Sidong Yang <sidong.yang@...iosa.ai>
Cc: Jens Axboe <axboe@...nel.dk>, Daniel Almeida <daniel.almeida@...labora.com>,
Benno Lossin <lossin@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, Arnd Bergmann <arnd@...db.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>, rust-for-linux@...r.kernel.org,
linux-kernel@...r.kernel.org, io-uring@...r.kernel.org
Subject: Re: [RFC PATCH v3 4/5] rust: miscdevice: Add `uring_cmd` support
On Fri, Aug 22, 2025 at 5:56 AM Sidong Yang <sidong.yang@...iosa.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@...iosa.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
>
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