[PATCH v2 1/2] rust: add scatterlist support

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

This patch adds a scatterlist abstraction. It is then used and tested
by the new rust virtio sample module.

Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
---
 rust/kernel/lib.rs         |  1 +
 rust/kernel/scatterlist.rs | 40 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)
 create mode 100644 rust/kernel/scatterlist.rs

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index c20b37e88ab2..b23be69919cc 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -71,6 +71,7 @@ pub mod net;
 pub mod pages;
 pub mod power;
 pub mod revocable;
+pub mod scatterlist;
 pub mod security;
 pub mod task;
 pub mod workqueue;
diff --git a/rust/kernel/scatterlist.rs b/rust/kernel/scatterlist.rs
new file mode 100644
index 000000000000..fe036af13995
--- /dev/null
+++ b/rust/kernel/scatterlist.rs
@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Scatterlist abstractions.
+//!
+//! C header: [`include/linux/virtio.h`](../../../../include/linux/scatterlist.h)
+
+use core::cell::UnsafeCell;
+
+/// Scatterlist abstraction over the C side `struct scatterlist`.
+pub struct Scatterlist {
+    /// The C side `struct scatterlist`.
+    inner: UnsafeCell<bindings::scatterlist>,
+}
+
+impl Scatterlist {
+    /// A wrapper over the C-side `sg_init_one()`. Initialize a single entry sg
+    /// list.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that `buf` is valid and `buflen` really
+    /// represents the size of `buf`.
+    pub unsafe fn init_one(buf: *const core::ffi::c_void, buflen: u32) -> Self {
+        // SAFETY: There are no references or function pointers in this
+        // `Scatterlist`.
+        let mut sg: Scatterlist = unsafe { core::mem::zeroed() };
+        // SAFETY: we pass a valid sg entry, which points to stack-allocated
+        // variable above. `buf` and `buflen` are presumed valid as per this
+        // function's safety requirements.
+        unsafe {
+            bindings::sg_init_one(sg.inner.get_mut(), buf, buflen);
+        }
+
+        sg
+    }
+
+    pub(crate) fn inner(&self) -> &UnsafeCell<bindings::scatterlist> {
+        &self.inner
+    }
+}
-- 
2.40.0