From b28e0506fafd9c987bba7a6a71ea02a37fcabdea Mon Sep 17 00:00:00 2001 From: "Michael S. Tsirkin" 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 Signed-off-by: Michael S. Tsirkin --- 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 61c818a7fe70..a3318f31e8e7 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. * -- 2.20.1