From: Michael S. Tsirkin Date: Thu, 25 Dec 2014 14:03:27 +0000 (+0200) Subject: virtio_ring: document alignment requirements X-Git-Tag: v3.19-rc3~7^2~1 X-Git-Url: http://git.cascardo.info/?p=cascardo%2Flinux.git;a=commitdiff_plain;h=b28e0506fafd9c987bba7a6a71ea02a37fcabdea 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 --- 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. *