io_uring: add support for user mapped provided buffer ring

The ring mapped provided buffer rings rely on the application allocating
the memory for the ring, and then the kernel will map it. This generally
works fine, but runs into issues on some architectures where we need
to be able to ensure that the kernel and application virtual address for
the ring play nicely together. This at least impacts architectures that
set SHM_COLOUR, but potentially also anyone setting SHMLBA.

To use this variant of ring provided buffers, the application need not
allocate any memory for the ring. Instead the kernel will do so, and
the allocation must subsequently call mmap(2) on the ring with the
offset set to:

	IORING_OFF_PBUF_RING | (bgid << IORING_OFF_PBUF_SHIFT)

to get a virtual address for the buffer ring. Normally the application
would allocate a suitable piece of memory (and correctly aligned) and
simply pass that in via io_uring_buf_reg.ring_addr and the kernel would
map it.

Outside of the setup differences, the kernel allocate + user mapped
provided buffer ring works exactly the same.

Acked-by: Helge Deller <deller@gmx.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 17 insertions, 0 deletions

diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index c3f3ea997f3a..1d59c816a5b8 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -389,6 +389,9 @@ enum {
 #define IORING_OFF_SQ_RING		0ULL
 #define IORING_OFF_CQ_RING		0x8000000ULL
 #define IORING_OFF_SQES			0x10000000ULL
+#define IORING_OFF_PBUF_RING		0x80000000ULL
+#define IORING_OFF_PBUF_SHIFT		16
+#define IORING_OFF_MMAP_MASK		0xf8000000ULL
 
 /*
  * Filled with the offset for mmap(2)
@@ -635,6 +638,20 @@ struct io_uring_buf_ring {
 	};
 };
 
+/*
+ * Flags for IORING_REGISTER_PBUF_RING.
+ *
+ * IOU_PBUF_RING_MMAP:	If set, kernel will allocate the memory for the ring.
+ *			The application must not set a ring_addr in struct
+ *			io_uring_buf_reg, instead it must subsequently call
+ *			mmap(2) with the offset set as:
+ *			IORING_OFF_PBUF_RING | (bgid << IORING_OFF_PBUF_SHIFT)
+ *			to get a virtual mapping for the ring.
+ */
+enum {
+	IOU_PBUF_RING_MMAP	= 1,
+};
+
 /* argument for IORING_(UN)REGISTER_PBUF_RING */
 struct io_uring_buf_reg {
 	__u64	ring_addr;