/*
 * Copyright Rusty Russell IBM Corporation 2007.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * $FreeBSD$
 */

#ifndef VIRTIO_RING_H
#define	VIRTIO_RING_H

#if defined __cplusplus
extern "C" {
#endif

/* This marks a buffer as continuing via the next field. */
#define VRING_DESC_F_NEXT       1
/* This marks a buffer as write-only (otherwise read-only). */
#define VRING_DESC_F_WRITE      2
/* This means the buffer contains a list of buffer descriptors. */
#define VRING_DESC_F_INDIRECT   4

/* The Host uses this in used->flags to advise the Guest: don't kick me
 * when you add a buffer.  It's unreliable, so it's simply an
 * optimization.  Guest will still kick if it's out of buffers.
 */
#define VRING_USED_F_NO_NOTIFY  1
/* The Guest uses this in avail->flags to advise the Host: don't
 * interrupt me when you consume a buffer.  It's unreliable, so it's
 * simply an optimization.
 */
#define VRING_AVAIL_F_NO_INTERRUPT      1

/* VirtIO ring descriptors: 16 bytes.
 * These can chain together via "next".
 */
struct vring_desc {
	/* Address (guest-physical). */
	uint64_t addr;
	/* Length. */
	uint32_t len;
	/* The flags as indicated above. */
	uint16_t flags;
	/* We chain unused descriptors via this, too. */
	uint16_t next;
};

struct vring_avail {
	uint16_t flags;
	uint16_t idx;
	uint16_t ring[0];
};

/* uint32_t is used here for ids for padding reasons. */
struct vring_used_elem {
	union {
		uint16_t event;
		/* Index of start of used descriptor chain. */
		uint32_t id;
	};
	/* Total length of the descriptor chain which was written to. */
	uint32_t len;
};

struct vring_used {
	uint16_t flags;
	uint16_t idx;
	struct vring_used_elem ring[0];
};

struct vring {
	unsigned int num;

	struct vring_desc *desc;
	struct vring_avail *avail;
	struct vring_used *used;
};

/* The standard layout for the ring is a continuous chunk of memory which
 * looks like this.  We assume num is a power of 2.
 *
 * struct vring {
 *      // The actual descriptors (16 bytes each)
 *      struct vring_desc desc[num];
 *
 *      // A ring of available descriptor heads with free-running index.
 *      __u16 avail_flags;
 *      __u16 avail_idx;
 *      __u16 available[num];
 *      __u16 used_event_idx;
 *
 *      // Padding to the next align boundary.
 *      char pad[];
 *
 *      // A ring of used descriptor heads with free-running index.
 *      __u16 used_flags;
 *      __u16 used_idx;
 *      struct vring_used_elem used[num];
 *      __u16 avail_event_idx;
 * };
 *
 * NOTE: for VirtIO PCI, align is 4096.
 */

/*
 * We publish the used event index at the end of the available ring, and vice
 * versa. They are at the end for backwards compatibility.
 */
#define vring_used_event(vr)	((vr)->avail->ring[(vr)->num])
#define vring_avail_event(vr)	((vr)->used->ring[(vr)->num].event)

static inline int vring_size(unsigned int num, unsigned long align)
{
	int size;

	size = num * sizeof(struct vring_desc);
	size += sizeof(struct vring_avail) + (num * sizeof(uint16_t)) +
	    sizeof(uint16_t);
	size = (size + align - 1) & ~(align - 1);
	size += sizeof(struct vring_used) +
	    (num * sizeof(struct vring_used_elem)) + sizeof(uint16_t);

	return size;
}

static inline void
vring_init(struct vring *vr, unsigned int num, uint8_t *p, unsigned long align)
{
	vr->num = num;
	vr->desc = (struct vring_desc *)p;
	vr->avail = (struct vring_avail *)(p + num * sizeof(struct vring_desc));
	vr->used = (struct vring_used *)
	    (((unsigned long)&vr->avail->ring[num] + sizeof(uint16_t) +
	      align - 1) & ~(align - 1));
}

/*
 * The following is used with VIRTIO_RING_F_EVENT_IDX.
 *
 * Assuming a given event_idx value from the other size, if we have
 * just incremented index from old to new_idx, should we trigger an
 * event?
 */
static inline int
vring_need_event(uint16_t event_idx, uint16_t new_idx, uint16_t old)
{
	return (uint16_t)(new_idx - event_idx - 1) <
	    (uint16_t)(new_idx - old);
}

#if defined __cplusplus
}
#endif

#endif				/* VIRTIO_RING_H */
