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virtio_ring.h
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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). */
65  /* Length. */
67  /* The flags as indicated above. */
69  /* We chain unused descriptors via this, too */
71 };
72 
73 struct vring_avail {
77 };
78 
79 /* u32 is used here for ids for padding reasons. */
81  /* Index of start of used descriptor chain. */
83  /* Total length of the descriptor chain which was used (written to) */
85 };
86 
87 struct vring_used {
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 */