1 #ifndef _LINUX_VIRTIO_RING_H
2 #define _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 <stdint.h>
35 #include "standard-headers/linux/types.h"
36 #include "standard-headers/linux/virtio_types.h"
37 
38 /* This marks a buffer as continuing via the next field. */
39 #define VRING_DESC_F_NEXT	1
40 /* This marks a buffer as write-only (otherwise read-only). */
41 #define VRING_DESC_F_WRITE	2
42 /* This means the buffer contains a list of buffer descriptors. */
43 #define VRING_DESC_F_INDIRECT	4
44 
45 /* The Host uses this in used->flags to advise the Guest: don't kick me when
46  * you add a buffer.  It's unreliable, so it's simply an optimization.  Guest
47  * will still kick if it's out of buffers. */
48 #define VRING_USED_F_NO_NOTIFY	1
49 /* The Guest uses this in avail->flags to advise the Host: don't interrupt me
50  * when you consume a buffer.  It's unreliable, so it's simply an
51  * optimization.  */
52 #define VRING_AVAIL_F_NO_INTERRUPT	1
53 
54 /* We support indirect buffer descriptors */
55 #define VIRTIO_RING_F_INDIRECT_DESC	28
56 
57 /* The Guest publishes the used index for which it expects an interrupt
58  * at the end of the avail ring. Host should ignore the avail->flags field. */
59 /* The Host publishes the avail index for which it expects a kick
60  * at the end of the used ring. Guest should ignore the used->flags field. */
61 #define VIRTIO_RING_F_EVENT_IDX		29
62 
63 /* Virtio ring descriptors: 16 bytes.  These can chain together via "next". */
64 struct vring_desc {
65 	/* Address (guest-physical). */
66 	__virtio64 addr;
67 	/* Length. */
68 	__virtio32 len;
69 	/* The flags as indicated above. */
70 	__virtio16 flags;
71 	/* We chain unused descriptors via this, too */
72 	__virtio16 next;
73 };
74 
75 struct vring_avail {
76 	__virtio16 flags;
77 	__virtio16 idx;
78 	__virtio16 ring[];
79 };
80 
81 /* u32 is used here for ids for padding reasons. */
82 struct vring_used_elem {
83 	/* Index of start of used descriptor chain. */
84 	__virtio32 id;
85 	/* Total length of the descriptor chain which was used (written to) */
86 	__virtio32 len;
87 };
88 
89 struct vring_used {
90 	__virtio16 flags;
91 	__virtio16 idx;
92 	struct vring_used_elem ring[];
93 };
94 
95 struct vring {
96 	unsigned int num;
97 
98 	struct vring_desc *desc;
99 
100 	struct vring_avail *avail;
101 
102 	struct vring_used *used;
103 };
104 
105 /* Alignment requirements for vring elements.
106  * When using pre-virtio 1.0 layout, these fall out naturally.
107  */
108 #define VRING_AVAIL_ALIGN_SIZE 2
109 #define VRING_USED_ALIGN_SIZE 4
110 #define VRING_DESC_ALIGN_SIZE 16
111 
112 /* The standard layout for the ring is a continuous chunk of memory which looks
113  * like this.  We assume num is a power of 2.
114  *
115  * struct vring
116  * {
117  *	// The actual descriptors (16 bytes each)
118  *	struct vring_desc desc[num];
119  *
120  *	// A ring of available descriptor heads with free-running index.
121  *	__virtio16 avail_flags;
122  *	__virtio16 avail_idx;
123  *	__virtio16 available[num];
124  *	__virtio16 used_event_idx;
125  *
126  *	// Padding to the next align boundary.
127  *	char pad[];
128  *
129  *	// A ring of used descriptor heads with free-running index.
130  *	__virtio16 used_flags;
131  *	__virtio16 used_idx;
132  *	struct vring_used_elem used[num];
133  *	__virtio16 avail_event_idx;
134  * };
135  */
136 /* We publish the used event index at the end of the available ring, and vice
137  * versa. They are at the end for backwards compatibility. */
138 #define vring_used_event(vr) ((vr)->avail->ring[(vr)->num])
139 #define vring_avail_event(vr) (*(__virtio16 *)&(vr)->used->ring[(vr)->num])
140 
141 static inline void vring_init(struct vring *vr, unsigned int num, void *p,
142 			      unsigned long align)
143 {
144 	vr->num = num;
145 	vr->desc = p;
146 	vr->avail = p + num*sizeof(struct vring_desc);
147 	vr->used = (void *)(((uintptr_t)&vr->avail->ring[num] + sizeof(__virtio16)
148 		+ align-1) & ~(align - 1));
149 }
150 
151 static inline unsigned vring_size(unsigned int num, unsigned long align)
152 {
153 	return ((sizeof(struct vring_desc) * num + sizeof(__virtio16) * (3 + num)
154 		 + align - 1) & ~(align - 1))
155 		+ sizeof(__virtio16) * 3 + sizeof(struct vring_used_elem) * num;
156 }
157 
158 /* The following is used with USED_EVENT_IDX and AVAIL_EVENT_IDX */
159 /* Assuming a given event_idx value from the other side, if
160  * we have just incremented index from old to new_idx,
161  * should we trigger an event? */
162 static inline int vring_need_event(uint16_t event_idx, uint16_t new_idx, uint16_t old)
163 {
164 	/* Note: Xen has similar logic for notification hold-off
165 	 * in include/xen/interface/io/ring.h with req_event and req_prod
166 	 * corresponding to event_idx + 1 and new_idx respectively.
167 	 * Note also that req_event and req_prod in Xen start at 1,
168 	 * event indexes in virtio start at 0. */
169 	return (uint16_t)(new_idx - event_idx - 1) < (uint16_t)(new_idx - old);
170 }
171 
172 #endif /* _LINUX_VIRTIO_RING_H */
173