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      1 #ifndef _UAPI_LINUX_VIRTIO_RING_H
      2 #define _UAPI_LINUX_VIRTIO_RING_H
      3 /* An interface for efficient virtio implementation, currently for use by KVM
      4  * and lguest, but hopefully others soon.  Do NOT change this since it will
      5  * break existing servers and clients.
      6  *
      7  * This header is BSD licensed so anyone can use the definitions to implement
      8  * compatible drivers/servers.
      9  *
     10  * Redistribution and use in source and binary forms, with or without
     11  * modification, are permitted provided that the following conditions
     12  * are met:
     13  * 1. Redistributions of source code must retain the above copyright
     14  *    notice, this list of conditions and the following disclaimer.
     15  * 2. Redistributions in binary form must reproduce the above copyright
     16  *    notice, this list of conditions and the following disclaimer in the
     17  *    documentation and/or other materials provided with the distribution.
     18  * 3. Neither the name of IBM nor the names of its contributors
     19  *    may be used to endorse or promote products derived from this software
     20  *    without specific prior written permission.
     21  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ``AS IS'' AND
     22  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     23  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     24  * ARE DISCLAIMED.  IN NO EVENT SHALL IBM OR CONTRIBUTORS BE LIABLE
     25  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     26  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     27  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     28  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     29  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     30  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     31  * SUCH DAMAGE.
     32  *
     33  * Copyright Rusty Russell IBM Corporation 2007. */
     34 #include <linux/types.h>
     35 
     36 /* This marks a buffer as continuing via the next field. */
     37 #define VRING_DESC_F_NEXT	1
     38 /* This marks a buffer as write-only (otherwise read-only). */
     39 #define VRING_DESC_F_WRITE	2
     40 /* This means the buffer contains a list of buffer descriptors. */
     41 #define VRING_DESC_F_INDIRECT	4
     42 
     43 /* The Host uses this in used->flags to advise the Guest: don't kick me when
     44  * you add a buffer.  It's unreliable, so it's simply an optimization.  Guest
     45  * will still kick if it's out of buffers. */
     46 #define VRING_USED_F_NO_NOTIFY	1
     47 /* The Guest uses this in avail->flags to advise the Host: don't interrupt me
     48  * when you consume a buffer.  It's unreliable, so it's simply an
     49  * optimization.  */
     50 #define VRING_AVAIL_F_NO_INTERRUPT	1
     51 
     52 /* We support indirect buffer descriptors */
     53 #define VIRTIO_RING_F_INDIRECT_DESC	28
     54 
     55 /* The Guest publishes the used index for which it expects an interrupt
     56  * at the end of the avail ring. Host should ignore the avail->flags field. */
     57 /* The Host publishes the avail index for which it expects a kick
     58  * at the end of the used ring. Guest should ignore the used->flags field. */
     59 #define VIRTIO_RING_F_EVENT_IDX		29
     60 
     61 /* Virtio ring descriptors: 16 bytes.  These can chain together via "next". */
     62 struct vring_desc {
     63 	/* Address (guest-physical). */
     64 	__u64 addr;
     65 	/* Length. */
     66 	__u32 len;
     67 	/* The flags as indicated above. */
     68 	__u16 flags;
     69 	/* We chain unused descriptors via this, too */
     70 	__u16 next;
     71 };
     72 
     73 struct vring_avail {
     74 	__u16 flags;
     75 	__u16 idx;
     76 	__u16 ring[];
     77 };
     78 
     79 /* u32 is used here for ids for padding reasons. */
     80 struct vring_used_elem {
     81 	/* Index of start of used descriptor chain. */
     82 	__u32 id;
     83 	/* Total length of the descriptor chain which was used (written to) */
     84 	__u32 len;
     85 };
     86 
     87 struct vring_used {
     88 	__u16 flags;
     89 	__u16 idx;
     90 	struct vring_used_elem ring[];
     91 };
     92 
     93 struct vring {
     94 	unsigned int num;
     95 
     96 	struct vring_desc *desc;
     97 
     98 	struct vring_avail *avail;
     99 
    100 	struct vring_used *used;
    101 };
    102 
    103 /* The standard layout for the ring is a continuous chunk of memory which looks
    104  * like this.  We assume num is a power of 2.
    105  *
    106  * struct vring
    107  * {
    108  *	// The actual descriptors (16 bytes each)
    109  *	struct vring_desc desc[num];
    110  *
    111  *	// A ring of available descriptor heads with free-running index.
    112  *	__u16 avail_flags;
    113  *	__u16 avail_idx;
    114  *	__u16 available[num];
    115  *	__u16 used_event_idx;
    116  *
    117  *	// Padding to the next align boundary.
    118  *	char pad[];
    119  *
    120  *	// A ring of used descriptor heads with free-running index.
    121  *	__u16 used_flags;
    122  *	__u16 used_idx;
    123  *	struct vring_used_elem used[num];
    124  *	__u16 avail_event_idx;
    125  * };
    126  */
    127 /* We publish the used event index at the end of the available ring, and vice
    128  * versa. They are at the end for backwards compatibility. */
    129 #define vring_used_event(vr) ((vr)->avail->ring[(vr)->num])
    130 #define vring_avail_event(vr) (*(__u16 *)&(vr)->used->ring[(vr)->num])
    131 
    132 static inline void vring_init(struct vring *vr, unsigned int num, void *p,
    133 			      unsigned long align)
    134 {
    135 	vr->num = num;
    136 	vr->desc = p;
    137 	vr->avail = p + num*sizeof(struct vring_desc);
    138 	vr->used = (void *)(((unsigned long)&vr->avail->ring[num] + sizeof(__u16)
    139 		+ align-1) & ~(align - 1));
    140 }
    141 
    142 static inline unsigned vring_size(unsigned int num, unsigned long align)
    143 {
    144 	return ((sizeof(struct vring_desc) * num + sizeof(__u16) * (3 + num)
    145 		 + align - 1) & ~(align - 1))
    146 		+ sizeof(__u16) * 3 + sizeof(struct vring_used_elem) * num;
    147 }
    148 
    149 /* The following is used with USED_EVENT_IDX and AVAIL_EVENT_IDX */
    150 /* Assuming a given event_idx value from the other size, if
    151  * we have just incremented index from old to new_idx,
    152  * should we trigger an event? */
    153 static inline int vring_need_event(__u16 event_idx, __u16 new_idx, __u16 old)
    154 {
    155 	/* Note: Xen has similar logic for notification hold-off
    156 	 * in include/xen/interface/io/ring.h with req_event and req_prod
    157 	 * corresponding to event_idx + 1 and new_idx respectively.
    158 	 * Note also that req_event and req_prod in Xen start at 1,
    159 	 * event indexes in virtio start at 0. */
    160 	return (__u16)(new_idx - event_idx - 1) < (__u16)(new_idx - old);
    161 }
    162 
    163 #endif /* _UAPI_LINUX_VIRTIO_RING_H */
    164