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.

Michael S. Tsirkin committed 11 years ago

Commit b28e0501 parent b7392d21 file · +7 −0Browse files

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include/uapi/linux/virtio_ring.h+7−0

@@ -101,6 +101,13 @@ struct vring { · 100 unmodified lines
101	struct vring_used *used;	101	struct vring_used *used;
102	};	102	};
103		103
		104	/* Alignment requirements for vring elements.
		105	* When using pre-virtio 1.0 layout, these fall out naturally.
		106	*/
		107	#define VRING_AVAIL_ALIGN_SIZE 2
		108	#define VRING_USED_ALIGN_SIZE 4
		109	#define VRING_DESC_ALIGN_SIZE 16
		110
104	/* The standard layout for the ring is a continuous chunk of memory which looks	111	/* The standard layout for the ring is a continuous chunk of memory which looks
105	* like this. We assume num is a power of 2.	112	* like this. We assume num is a power of 2.
106	*	113	*