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virtio_ring: document alignment requirements

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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>
This commit is contained in:
Michael S. Tsirkin 2014-12-25 16:03:27 +02:00
parent b7392d2247
commit b28e0506fa
1 changed files with 7 additions and 0 deletions
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7
include/uapi/linux/virtio_ring.h
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@ -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.
*