From b75729b339aeeb232c0b9dff0ed4caf5633695e7 Mon Sep 17 00:00:00 2001
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: Thu, 25 Dec 2014 16:03:27 +0200
Subject: [PATCH] virtio_ring: document alignment requirements
Host needs to know vring element alignment requirements:
simply doing alignof on structures doesn't work reliably: on some
platforms gcc has alignof(uint32_t) == 2.
Add macros for alignment as specified in virtio 1.0 cs01,
export them to userspace as well.
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
include/uapi/linux/virtio_ring.h | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h
index 61c818a7fe70d..a3318f31e8e7f 100644
--- a/include/uapi/linux/virtio_ring.h
+++ b/include/uapi/linux/virtio_ring.h
@@ -101,6 +101,13 @@ struct vring {
struct vring_used *used;
};
+/* Alignment requirements for vring elements.
+ * When using pre-virtio 1.0 layout, these fall out naturally.
+ */
+#define VRING_AVAIL_ALIGN_SIZE 2
+#define VRING_USED_ALIGN_SIZE 4
+#define VRING_DESC_ALIGN_SIZE 16
+
/* The standard layout for the ring is a continuous chunk of memory which looks
* like this. We assume num is a power of 2.
*
--
2.39.5