[RFC PATCH v3 3/5] rust: io_uring: introduce rust abstraction for io-uring cmd

[Sidong Yang <sidong.yang@furiosa.ai>]

Implment the io-uring abstractions needed for miscdevicecs and other
char devices that have io-uring command interface.

* `io_uring::IoUringCmd` : Rust abstraction for `io_uring_cmd` which
  will be used as arg for `MiscDevice::uring_cmd()`. And driver can get
  `cmd_op` sent from userspace. Also it has `flags` which includes option
  that is reissued.

* `io_uring::IoUringSqe` : Rust abstraction for `io_uring_sqe` which
  could be get from `IoUringCmd::sqe()` and driver could get `cmd_data`
  from userspace. Also `IoUringSqe` has more data like opcode could be used in
  driver.

Signed-off-by: Sidong Yang <sidong.yang@furiosa.ai>
---
 rust/kernel/io_uring.rs | 306 ++++++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs      |   1 +
 2 files changed, 307 insertions(+)
 create mode 100644 rust/kernel/io_uring.rs

diff --git a/rust/kernel/io_uring.rs b/rust/kernel/io_uring.rs
new file mode 100644
index 000000000000..61e88bdf4e42
--- /dev/null
+++ b/rust/kernel/io_uring.rs
@@ -0,0 +1,306 @@
+// SPDX-License-Identifier: GPL-2.0
+// SPDX-FileCopyrightText: (C) 2025 Furiosa AI
+
+//! Abstractions for io-uring.
+//!
+//! This module provides types for implements io-uring interface for char device.
+//!
+//!
+//! C headers: [`include/linux/io_uring/cmd.h`](srctree/include/linux/io_uring/cmd.h) and
+//! [`include/linux/io_uring/io_uring.h`](srctree/include/linux/io_uring/io_uring.h)
+
+use core::{mem::MaybeUninit, pin::Pin};
+
+use crate::error::from_result;
+use crate::transmute::{AsBytes, FromBytes};
+use crate::{fs::File, types::Opaque};
+
+use crate::prelude::*;
+
+/// io-uring opcode
+pub mod opcode {
+    /// opcode for uring cmd
+    pub const URING_CMD: u32 = bindings::io_uring_op_IORING_OP_URING_CMD;
+}
+
+/// A Rust abstraction for the Linux kernel's `io_uring_cmd` structure.
+///
+/// This structure is a safe, opaque wrapper around the raw C `io_uring_cmd`
+/// binding from the Linux kernel. It represents a command structure used
+/// in io_uring operations within the kernel.
+/// This type is used internally by the io_uring subsystem to manage
+/// asynchronous I/O commands.
+///
+/// This type should not be constructed or manipulated directly by
+/// kernel module developers.
+///
+/// # INVARIANT
+/// - `self.inner` always points to a valid, live `bindings::io_uring_cmd`.
+#[repr(transparent)]
+pub struct IoUringCmd {
+    /// An opaque wrapper containing the actual `io_uring_cmd` data.
+    inner: Opaque<bindings::io_uring_cmd>,
+}
+
+impl IoUringCmd {
+    /// Returns the cmd_op with associated with the `io_uring_cmd`.
+    #[inline]
+    pub fn cmd_op(&self) -> u32 {
+        // SAFETY: `self.inner` is guaranteed by the type invariant to point
+        // to a live `io_uring_cmd`, so dereferencing is safe.
+        unsafe { (*self.inner.get()).cmd_op }
+    }
+
+    /// Returns the flags with associated with the `io_uring_cmd`.
+    #[inline]
+    pub fn flags(&self) -> u32 {
+        // SAFETY: `self.inner` is guaranteed by the type invariant to point
+        // to a live `io_uring_cmd`, so dereferencing is safe.
+        unsafe { (*self.inner.get()).flags }
+    }
+
+    /// Reads protocol data unit as `T` that impl `FromBytes` from uring cmd
+    ///
+    /// Fails with [`EFAULT`] if size of `T` is bigger than pdu size.
+    #[inline]
+    pub fn read_pdu<T: FromBytes>(&self) -> Result<T> {
+        // SAFETY: `self.inner` is guaranteed by the type invariant to point
+        // to a live `io_uring_cmd`, so dereferencing is safe.
+        let inner = unsafe { &mut *self.inner.get() };
+
+        let len = size_of::<T>();
+        if len > inner.pdu.len() {
+            return Err(EFAULT);
+        }
+
+        let mut out: MaybeUninit<T> = MaybeUninit::uninit();
+        let ptr = &raw mut inner.pdu as *const c_void;
+
+        // SAFETY:
+        // * The `ptr` is valid pointer from `self.inner` that is guaranteed by type invariant.
+        // * The `out` is valid pointer that points `T` which impls `FromBytes` and checked
+        //   size of `T` is smaller than pdu size.
+        unsafe {
+            core::ptr::copy_nonoverlapping(ptr, out.as_mut_ptr().cast::<c_void>(), len);
+        }
+
+        // SAFETY: The read above has initialized all bytes in `out`, and since `T` implements
+        // `FromBytes`, any bit-pattern is a valid value for this type.
+        Ok(unsafe { out.assume_init() })
+    }
+
+    /// Writes the provided `value` to `pdu` in uring_cmd `self`
+    ///
+    /// Fails with [`EFAULT`] if size of `T` is bigger than pdu size.
+    #[inline]
+    pub fn write_pdu<T: AsBytes>(&mut self, value: &T) -> Result<()> {
+        // SAFETY: `self.inner` is guaranteed by the type invariant to point
+        // to a live `io_uring_cmd`, so dereferencing is safe.
+        let inner = unsafe { &mut *self.inner.get() };
+
+        let len = size_of::<T>();
+        if len > inner.pdu.len() {
+            return Err(EFAULT);
+        }
+
+        let src = (value as *const T).cast::<c_void>();
+        let dst = &raw mut inner.pdu as *mut c_void;
+
+        // SAFETY:
+        // * The `src` is points valid memory that is guaranteed by `T` impls `AsBytes`
+        // * The `dst` is valid. It's from `self.inner` that is guaranteed by type invariant.
+        // * It's safe to copy because size of `T` is no more than len of pdu.
+        unsafe {
+            core::ptr::copy_nonoverlapping(src, dst, len);
+        }
+
+        Ok(())
+    }
+
+    /// Constructs a new [`IoUringCmd`] from a raw `io_uring_cmd`
+    ///
+    /// # Safety
+    ///
+    /// The caller must guarantee that:
+    /// - `ptr` is non-null, properly aligned, and points to a valid
+    ///   `bindings::io_uring_cmd`.
+    /// - The pointed-to memory remains initialized and valid for the entire
+    ///   lifetime `'a` of the returned reference.
+    /// - While the returned `Pin<&'a mut IoUringCmd>` is alive, the underlying
+    ///   object is **not moved** (pinning requirement).
+    /// - **Aliasing rules:** the returned `&mut` has **exclusive** access to the same
+    ///   object for its entire lifetime:
+    ///   - No other `&mut` **or** `&` references to the same `io_uring_cmd` may be
+    ///     alive at the same time.
+    ///   - There must be no concurrent reads/writes through raw pointers, FFI, or
+    ///     other kernel paths to the same object during this lifetime.
+    ///   - If the object can be touched from other contexts (e.g. IRQ/another CPU),
+    ///     the caller must provide synchronization to uphold this exclusivity.
+    /// - This function relies on `IoUringCmd` being `repr(transparent)` over
+    ///   `bindings::io_uring_cmd` so the cast preserves layout.
+    #[inline]
+    pub unsafe fn from_raw<'a>(ptr: *mut bindings::io_uring_cmd) -> Pin<&'a mut IoUringCmd> {
+        // SAFETY:
+        // * The caller guarantees that the pointer is not dangling and stays
+        //   valid for the duration of 'a.
+        // * The cast is okay because `IoUringCmd` is `repr(transparent)` and
+        //   has the same memory layout as `bindings::io_uring_cmd`.
+        // * The returned `Pin` ensures that the object cannot be moved, which
+        //   is required because the kernel may hold pointers to this memory
+        //   location and moving it would invalidate those pointers.
+        unsafe { Pin::new_unchecked(&mut *ptr.cast()) }
+    }
+
+    /// Returns the file that referenced by uring cmd self.
+    #[inline]
+    pub fn file(&self) -> &File {
+        // SAFETY: `self.inner` is guaranteed by the type invariant to point
+        // to a live `io_uring_cmd`, so dereferencing is safe.
+        let file = unsafe { (*self.inner.get()).file };
+
+        // SAFETY:
+        // * The `file` points valid file.
+        // * refcount is positive after submission queue entry issued.
+        // * There is no active fdget_pos region on the file on this thread.
+        unsafe { File::from_raw_file(file) }
+    }
+
+    /// Returns an reference to the [`IoUringSqe`] associated with this command.
+    #[inline]
+    pub fn sqe(&self) -> &IoUringSqe {
+        // SAFETY: `self.inner` is guaranteed by the type invariant to point
+        // to a live `io_uring_cmd`, so dereferencing is safe.
+        let sqe = unsafe { (*self.inner.get()).sqe };
+        // SAFETY: The call guarantees that the `sqe` points valid io_uring_sqe.
+        unsafe { IoUringSqe::from_raw(sqe) }
+    }
+
+    /// Completes an this [`IoUringCmd`] request that was previously queued.
+    ///
+    /// # Safety
+    ///
+    /// - This function must be called **only** for a command whose `uring_cmd`
+    ///   handler previously returned **`-EIOCBQUEUED`** to io_uring.
+    ///
+    /// # Parameters
+    ///
+    /// - `ret`: Result to return to userspace.
+    /// - `res2`: Extra for big completion queue entry `IORING_SETUP_CQE32`.
+    /// - `issue_flags`: Flags associated with this request, typically the same
+    ///   as those passed to the `uring_cmd` handler.
+    #[inline]
+    pub fn done(self: Pin<&mut IoUringCmd>, ret: Result<i32>, res2: u64, issue_flags: u32) {
+        let ret = from_result(|| ret) as isize;
+        // SAFETY: The call guarantees that `self.inner` is not dangling and stays valid
+        unsafe {
+            bindings::io_uring_cmd_done(self.inner.get(), ret, res2, issue_flags);
+        }
+    }
+}
+
+/// A Rust abstraction for the Linux kernel's `io_uring_sqe` structure.
+///
+/// This structure is a safe, opaque wrapper around the raw C [`io_uring_sqe`](srctree/include/uapi/linux/io_uring.h)
+/// binding from the Linux kernel. It represents a Submission Queue Entry
+/// used in io_uring operations within the kernel.
+///
+/// # Type Safety
+///
+/// The `#[repr(transparent)]` attribute ensures that this wrapper has
+/// the same memory layout as the underlying `io_uring_sqe` structure,
+/// allowing it to be safely transmuted between the two representations.
+///
+/// # Fields
+///
+/// * `inner` - An opaque wrapper containing the actual `io_uring_sqe` data.
+///             The `Opaque` type prevents direct access to the internal
+///             structure fields, ensuring memory safety and encapsulation.
+///
+/// # Usage
+///
+/// This type represents a submission queue entry that describes an I/O
+/// operation to be executed by the io_uring subsystem. It contains
+/// information such as the operation type, file descriptor, buffer
+/// pointers, and other operation-specific data.
+///
+/// Users can obtain this type from [`IoUringCmd::sqe()`] method, which
+/// extracts the submission queue entry associated with a command.
+///
+/// This type should not be constructed or manipulated directly by
+/// kernel module developers.
+///
+/// # INVARIANT
+/// - `self.inner` always points to a valid, live `bindings::io_uring_sqe`.
+#[repr(transparent)]
+pub struct IoUringSqe {
+    inner: Opaque<bindings::io_uring_sqe>,
+}
+
+impl IoUringSqe {
+    /// Reads and interprets the `cmd` field of an `bindings::io_uring_sqe` as a value of type `T`.
+    ///
+    /// # Safety & Invariants
+    /// - Construction of `T` is delegated to `FromBytes`, which guarantees that `T` has no
+    ///   invalid bit patterns and can be safely reconstructed from raw bytes.
+    /// - **Limitation:** This implementation does not support `IORING_SETUP_SQE128` (larger SQE entries).
+    ///   Only the standard `io_uring_sqe` layout is handled here.
+    ///
+    /// # Errors
+    /// * Returns `EINVAL` if the `self` does not hold a `opcode::URING_CMD`.
+    /// * Returns `EFAULT` if the command buffer is smaller than the requested type `T`.
+    ///
+    /// # Returns
+    /// * On success, returns a `T` deserialized from the `cmd`.
+    /// * On failure, returns an appropriate error as described above.
+    pub fn cmd_data<T: FromBytes>(&self) -> Result<T> {
+        // SAFETY: `self.inner` guaranteed by the type invariant to point
+        // to a live `io_uring_sqe`, so dereferencing is safe.
+        let sqe = unsafe { &*self.inner.get() };
+
+        if u32::from(sqe.opcode) != opcode::URING_CMD {
+            return Err(EINVAL);
+        }
+
+        // SAFETY: Accessing the `sqe.cmd` union field is safe because we've
+        // verified that `sqe.opcode == IORING_OP_URING_CMD`, which guarantees
+        // that this union variant is initialized and valid.
+        let cmd = unsafe { sqe.__bindgen_anon_6.cmd.as_ref() };
+        let cmd_len = size_of_val(&sqe.__bindgen_anon_6.bindgen_union_field);
+
+        if cmd_len < size_of::<T>() {
+            return Err(EFAULT);
+        }
+
+        let cmd_ptr = cmd.as_ptr() as *mut T;
+
+        // SAFETY: `cmd_ptr` is valid from `self.inner` which is guaranteed by
+        // type variant. And also it points to initialized `T` from userspace.
+        let ret = unsafe { core::ptr::read_unaligned(cmd_ptr) };
+
+        Ok(ret)
+    }
+
+    /// Constructs a new `IoUringSqe` from a raw `io_uring_sqe`.
+    ///
+    /// # Safety
+    ///
+    /// The caller must guarantee that:
+    /// - `ptr` is non-null, properly aligned, and points to a valid initialized
+    ///   `bindings::io_uring_sqe`.
+    /// - The pointed-to memory remains valid (not freed or repurposed) for the
+    ///   entire lifetime `'a` of the returned reference.
+    /// - **Aliasing rules (for `&T`):** while the returned `&'a IoUringSqe` is
+    ///   alive, there must be **no mutable access** to the same object through any
+    ///   path (no `&mut`, no raw-pointer writes, no FFI/IRQ/other-CPU writers).
+    ///   Multiple `&` is fine **only if all of them are read-only** for the entire
+    ///   overlapping lifetime.
+    /// - This relies on `IoUringSqe` being `repr(transparent)` over
+    ///   `bindings::io_uring_sqe`, so the cast preserves layout.
+    #[inline]
+    pub unsafe fn from_raw<'a>(ptr: *const bindings::io_uring_sqe) -> &'a IoUringSqe {
+        // SAFETY: The caller guarantees that the pointer is not dangling and stays valid for the
+        // duration of 'a. The cast is okay because `IoUringSqe` is `repr(transparent)` and has the
+        // same memory layout as `bindings::io_uring_sqe`.
+        unsafe { &*ptr.cast() }
+    }
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index ed53169e795c..d38cf7137401 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -91,6 +91,7 @@
 pub mod fs;
 pub mod init;
 pub mod io;
+pub mod io_uring;
 pub mod ioctl;
 pub mod jump_label;
 #[cfg(CONFIG_KUNIT)]
-- 
2.43.0