virtio_ring.h

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/*
 * Copyright Rusty Russell IBM Corporation 2007.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * $FreeBSD$
 */
 
#ifndef VIRTIO_RING_H
#define VIRTIO_RING_H
 
#include <metal/compiler.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".
 */
METAL_PACKED_BEGIN
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;
} METAL_PACKED_END;
 
METAL_PACKED_BEGIN
struct vring_avail {
        uint16_t flags;
        uint16_t idx;
        uint16_t ring[0];
} METAL_PACKED_END;
 
/* uint32_t is used here for ids for padding reasons. */
METAL_PACKED_BEGIN
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;
} METAL_PACKED_END;
 
METAL_PACKED_BEGIN
struct vring_used {
        uint16_t flags;
        uint16_t idx;
        struct vring_used_elem ring[0];
} METAL_PACKED_END;
 
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 */