vring_used
uint16_t hdrlen = offsetof(struct vring_used, ring);
uint16_t hdrlen = offsetof(struct vring_used, ring);
uint16_t hdrlen = offsetof(struct vring_used, ring);
uint16_t hdrlen = offsetof(struct vring_used, ring);
offsetof(struct vring_used, ring[vq_num]));
struct vring_used *vq_used;