include/standard-headers/linux/virtio_ring.h

   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  * 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 /*
  46  * Mark a descriptor as available or used in packed ring.
  47  * Notice: they are defined as shifts instead of shifted values.
  48  */
  49 #define VRING_PACKED_DESC_F_AVAIL       7
  50 #define VRING_PACKED_DESC_F_USED        15
  51
  52 /* The Host uses this in used->flags to advise the Guest: don't kick me when
  53  * you add a buffer.  It's unreliable, so it's simply an optimization.  Guest
  54  * will still kick if it's out of buffers. */
  55 #define VRING_USED_F_NO_NOTIFY  1
  56 /* The Guest uses this in avail->flags to advise the Host: don't interrupt me
  57  * when you consume a buffer.  It's unreliable, so it's simply an
  58  * optimization.  */
  59 #define VRING_AVAIL_F_NO_INTERRUPT      1
  60
  61 /* Enable events in packed ring. */
  62 #define VRING_PACKED_EVENT_FLAG_ENABLE  0x0
  63 /* Disable events in packed ring. */
  64 #define VRING_PACKED_EVENT_FLAG_DISABLE 0x1
  65 /*
  66  * Enable events for a specific descriptor in packed ring.
  67  * (as specified by Descriptor Ring Change Event Offset/Wrap Counter).
  68  * Only valid if VIRTIO_RING_F_EVENT_IDX has been negotiated.
  69  */
  70 #define VRING_PACKED_EVENT_FLAG_DESC    0x2
  71
  72 /*
  73  * Wrap counter bit shift in event suppression structure
  74  * of packed ring.
  75  */
  76 #define VRING_PACKED_EVENT_F_WRAP_CTR   15
  77
  78 /* We support indirect buffer descriptors */
  79 #define VIRTIO_RING_F_INDIRECT_DESC     28
  80
  81 /* The Guest publishes the used index for which it expects an interrupt
  82  * at the end of the avail ring. Host should ignore the avail->flags field. */
  83 /* The Host publishes the avail index for which it expects a kick
  84  * at the end of the used ring. Guest should ignore the used->flags field. */
  85 #define VIRTIO_RING_F_EVENT_IDX         29
  86
  87 /* Alignment requirements for vring elements.
  88  * When using pre-virtio 1.0 layout, these fall out naturally.
  89  */
  90 #define VRING_AVAIL_ALIGN_SIZE 2
  91 #define VRING_USED_ALIGN_SIZE 4
  92 #define VRING_DESC_ALIGN_SIZE 16
  93
  94 /**
  95  * struct vring_desc - Virtio ring descriptors,
  96  * 16 bytes long. These can chain together via @next.
  97  *
  98  * @addr: buffer address (guest-physical)
  99  * @len: buffer length
 100  * @flags: descriptor flags
 101  * @next: index of the next descriptor in the chain,
 102  *        if the VRING_DESC_F_NEXT flag is set. We chain unused
 103  *        descriptors via this, too.
 104  */
 105 struct vring_desc {
 106         __virtio64 addr;
 107         __virtio32 len;
 108         __virtio16 flags;
 109         __virtio16 next;
 110 };
 111
 112 struct vring_avail {
 113         __virtio16 flags;
 114         __virtio16 idx;
 115         __virtio16 ring[];
 116 };
 117
 118 /* uint32_t is used here for ids for padding reasons. */
 119 struct vring_used_elem {
 120         /* Index of start of used descriptor chain. */
 121         __virtio32 id;
 122         /* Total length of the descriptor chain which was used (written to) */
 123         __virtio32 len;
 124 };
 125
 126 typedef struct vring_used_elem __attribute__((aligned(VRING_USED_ALIGN_SIZE)))
 127         vring_used_elem_t;
 128
 129 struct vring_used {
 130         __virtio16 flags;
 131         __virtio16 idx;
 132         vring_used_elem_t ring[];
 133 };
 134
 135 /*
 136  * The ring element addresses are passed between components with different
 137  * alignments assumptions. Thus, we might need to decrease the compiler-selected
 138  * alignment, and so must use a typedef to make sure the aligned attribute
 139  * actually takes hold:
 140  *
 141  * https://gcc.gnu.org/onlinedocs//gcc/Common-Type-Attributes.html#Common-Type-Attributes
 142  *
 143  * When used on a struct, or struct member, the aligned attribute can only
 144  * increase the alignment; in order to decrease it, the packed attribute must
 145  * be specified as well. When used as part of a typedef, the aligned attribute
 146  * can both increase and decrease alignment, and specifying the packed
 147  * attribute generates a warning.
 148  */
 149 typedef struct vring_desc __attribute__((aligned(VRING_DESC_ALIGN_SIZE)))
 150         vring_desc_t;
 151 typedef struct vring_avail __attribute__((aligned(VRING_AVAIL_ALIGN_SIZE)))
 152         vring_avail_t;
 153 typedef struct vring_used __attribute__((aligned(VRING_USED_ALIGN_SIZE)))
 154         vring_used_t;
 155
 156 struct vring {
 157         unsigned int num;
 158
 159         vring_desc_t *desc;
 160
 161         vring_avail_t *avail;
 162
 163         vring_used_t *used;
 164 };
 165
 166 #ifndef VIRTIO_RING_NO_LEGACY
 167
 168 /* The standard layout for the ring is a continuous chunk of memory which looks
 169  * like this.  We assume num is a power of 2.
 170  *
 171  * struct vring
 172  * {
 173  *      // The actual descriptors (16 bytes each)
 174  *      struct vring_desc desc[num];
 175  *
 176  *      // A ring of available descriptor heads with free-running index.
 177  *      __virtio16 avail_flags;
 178  *      __virtio16 avail_idx;
 179  *      __virtio16 available[num];
 180  *      __virtio16 used_event_idx;
 181  *
 182  *      // Padding to the next align boundary.
 183  *      char pad[];
 184  *
 185  *      // A ring of used descriptor heads with free-running index.
 186  *      __virtio16 used_flags;
 187  *      __virtio16 used_idx;
 188  *      struct vring_used_elem used[num];
 189  *      __virtio16 avail_event_idx;
 190  * };
 191  */
 192 /* We publish the used event index at the end of the available ring, and vice
 193  * versa. They are at the end for backwards compatibility. */
 194 #define vring_used_event(vr) ((vr)->avail->ring[(vr)->num])
 195 #define vring_avail_event(vr) (*(__virtio16 *)&(vr)->used->ring[(vr)->num])
 196
 197 static inline void vring_init(struct vring *vr, unsigned int num, void *p,
 198                               unsigned long align)
 199 {
 200         vr->num = num;
 201         vr->desc = p;
 202         vr->avail = (struct vring_avail *)((char *)p + num * sizeof(struct vring_desc));
 203         vr->used = (void *)(((uintptr_t)&vr->avail->ring[num] + sizeof(__virtio16)
 204                 + align-1) & ~(align - 1));
 205 }
 206
 207 static inline unsigned vring_size(unsigned int num, unsigned long align)
 208 {
 209         return ((sizeof(struct vring_desc) * num + sizeof(__virtio16) * (3 + num)
 210                  + align - 1) & ~(align - 1))
 211                 + sizeof(__virtio16) * 3 + sizeof(struct vring_used_elem) * num;
 212 }
 213
 214 #endif /* VIRTIO_RING_NO_LEGACY */
 215
 216 /* The following is used with USED_EVENT_IDX and AVAIL_EVENT_IDX */
 217 /* Assuming a given event_idx value from the other side, if
 218  * we have just incremented index from old to new_idx,
 219  * should we trigger an event? */
 220 static inline int vring_need_event(uint16_t event_idx, uint16_t new_idx, uint16_t old)
 221 {
 222         /* Note: Xen has similar logic for notification hold-off
 223          * in include/xen/interface/io/ring.h with req_event and req_prod
 224          * corresponding to event_idx + 1 and new_idx respectively.
 225          * Note also that req_event and req_prod in Xen start at 1,
 226          * event indexes in virtio start at 0. */
 227         return (uint16_t)(new_idx - event_idx - 1) < (uint16_t)(new_idx - old);
 228 }
 229
 230 struct vring_packed_desc_event {
 231         /* Descriptor Ring Change Event Offset/Wrap Counter. */
 232         uint16_t off_wrap;
 233         /* Descriptor Ring Change Event Flags. */
 234         uint16_t flags;
 235 };
 236
 237 struct vring_packed_desc {
 238         /* Buffer Address. */
 239         uint64_t addr;
 240         /* Buffer Length. */
 241         uint32_t len;
 242         /* Buffer ID. */
 243         uint16_t id;
 244         /* The flags depending on descriptor type. */
 245         uint16_t flags;
 246 };
 247
 248 #endif /* _LINUX_VIRTIO_RING_H */