[PATCH v4 5/5] io_uring/rsrc: add io_uring_registered_mem_region_get()

[Joanne Koong <joannelkoong@gmail.com>]

Add io_uring_registered_mem_region_get() helper to allow io_uring
command handlers to retrieve the ring's registered memory region info
(the vmapped pointer to the region's pages and the size of the region).
The info is returned in a struct io_uring_mem_region_info. If no region
was registered, a zeroed struct is returned. This provides a way for
uring cmd implementations to directly access pre-registered memory for
passing data.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 include/linux/io_uring/cmd.h | 14 ++++++++++++++
 io_uring/rsrc.c              | 25 +++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/include/linux/io_uring/cmd.h b/include/linux/io_uring/cmd.h
index b4d1d5e8e851..f1027f357ad1 100644
--- a/include/linux/io_uring/cmd.h
+++ b/include/linux/io_uring/cmd.h
@@ -20,6 +20,11 @@ struct io_uring_cmd {
 	u8		unused[8];
 };
 
+struct io_uring_mem_region_info {
+	void		*ptr;
+	unsigned	nr_pages;
+};
+
 #define io_uring_sqe128_cmd(sqe, type)	({					\
 	BUILD_BUG_ON(sizeof(type) > ((2 * sizeof(struct io_uring_sqe)) -	\
 				     offsetof(struct io_uring_sqe, cmd)));	\
@@ -51,6 +56,9 @@ int io_uring_cmd_import_fixed_vec(struct io_uring_cmd *ioucmd,
 				  size_t uvec_segs,
 				  int ddir, struct iov_iter *iter,
 				  unsigned issue_flags);
+struct io_uring_mem_region_info
+io_uring_registered_mem_region_get(struct io_uring_cmd *cmd,
+				   unsigned issue_flags);
 
 /*
  * Completes the request, i.e. posts an io_uring CQE and deallocates @ioucmd
@@ -132,6 +140,12 @@ static inline int io_uring_cmd_import_fixed_vec(struct io_uring_cmd *ioucmd,
 {
 	return -EOPNOTSUPP;
 }
+static inline struct io_uring_mem_region_info
+io_uring_registered_mem_region_get(struct io_uring_cmd *cmd,
+				   unsigned issue_flags)
+{
+	return (struct io_uring_mem_region_info) {};
+}
 static inline void __io_uring_cmd_done(struct io_uring_cmd *cmd, s32 ret,
 		u64 ret2, unsigned issue_flags, bool is_cqe32)
 {
diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
index cf5638406a0c..7b084823ef68 100644
--- a/io_uring/rsrc.c
+++ b/io_uring/rsrc.c
@@ -1182,6 +1182,31 @@ int io_import_reg_buf(struct io_kiocb *req, struct iov_iter *iter,
 	return io_import_fixed(ddir, iter, node->buf, buf_addr, len);
 }
 
+struct io_uring_mem_region_info
+io_uring_registered_mem_region_get(struct io_uring_cmd *cmd,
+				   unsigned issue_flags)
+{
+	struct io_ring_ctx *ctx = cmd_to_io_kiocb(cmd)->ctx;
+	struct io_uring_mem_region_info info = {};
+
+	/*
+	 * The submit lock ensures we don't see partially initialized state if
+	 * another thread is currently registering the region. Once registered,
+	 * the region is stable for the ring's lifetime (no unregister API
+	 * exists), so it's safe to access the returned pointer outside the
+	 * lock.
+	 */
+	io_ring_submit_lock(ctx, issue_flags);
+
+	info.ptr = ctx->param_region.ptr;
+	info.nr_pages = ctx->param_region.nr_pages;
+
+	io_ring_submit_unlock(ctx, issue_flags);
+
+	return info;
+}
+EXPORT_SYMBOL_GPL(io_uring_registered_mem_region_get);
+
 /* Lock two rings at once. The rings must be different! */
 static void lock_two_rings(struct io_ring_ctx *ctx1, struct io_ring_ctx *ctx2)
 {
-- 
2.52.0