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 /* Virtio ring descriptors: 16 bytes.  These can chain together via "next". */
  95 struct vring_desc {
  96         /* Address (guest-physical). */
  97         __virtio64 addr;
  98         /* Length. */
  99         __virtio32 len;
 100         /* The flags as indicated above. */
 101         __virtio16 flags;
 102         /* We chain unused descriptors via this, too */
 103         __virtio16 next;
 104 };
 105
 106 struct vring_avail {
 107         __virtio16 flags;
 108         __virtio16 idx;
 109         __virtio16 ring[];
 110 };
 111
 112 /* uint32_t is used here for ids for padding reasons. */
 113 struct vring_used_elem {
 114         /* Index of start of used descriptor chain. */
 115         __virtio32 id;
 116         /* Total length of the descriptor chain which was used (written to) */
 117         __virtio32 len;
 118 };
 119
 120 typedef struct vring_used_elem __attribute__((aligned(VRING_USED_ALIGN_SIZE)))
 121         vring_used_elem_t;
 122
 123 struct vring_used {
 124         __virtio16 flags;
 125         __virtio16 idx;
 126         vring_used_elem_t ring[];
 127 };
 128
 129 /*
 130  * The ring element addresses are passed between components with different
 131  * alignments assumptions. Thus, we might need to decrease the compiler-selected
 132  * alignment, and so must use a typedef to make sure the aligned attribute
 133  * actually takes hold:
 134  *
 135  * https://gcc.gnu.org/onlinedocs//gcc/Common-Type-Attributes.html#Common-Type-Attributes
 136  *
 137  * When used on a struct, or struct member, the aligned attribute can only
 138  * increase the alignment; in order to decrease it, the packed attribute must
 139  * be specified as well. When used as part of a typedef, the aligned attribute
 140  * can both increase and decrease alignment, and specifying the packed
 141  * attribute generates a warning.
 142  */
 143 typedef struct vring_desc __attribute__((aligned(VRING_DESC_ALIGN_SIZE)))
 144         vring_desc_t;
 145 typedef struct vring_avail __attribute__((aligned(VRING_AVAIL_ALIGN_SIZE)))
 146         vring_avail_t;
 147 typedef struct vring_used __attribute__((aligned(VRING_USED_ALIGN_SIZE)))
 148         vring_used_t;
 149
 150 struct vring {
 151         unsigned int num;
 152
 153         vring_desc_t *desc;
 154
 155         vring_avail_t *avail;
 156
 157         vring_used_t *used;
 158 };
 159
 160 #ifndef VIRTIO_RING_NO_LEGACY
 161
 162 /* The standard layout for the ring is a continuous chunk of memory which looks
 163  * like this.  We assume num is a power of 2.
 164  *
 165  * struct vring
 166  * {
 167  *      // The actual descriptors (16 bytes each)
 168  *      struct vring_desc desc[num];
 169  *
 170  *      // A ring of available descriptor heads with free-running index.
 171  *      __virtio16 avail_flags;
 172  *      __virtio16 avail_idx;
 173  *      __virtio16 available[num];
 174  *      __virtio16 used_event_idx;
 175  *
 176  *      // Padding to the next align boundary.
 177  *      char pad[];
 178  *
 179  *      // A ring of used descriptor heads with free-running index.
 180  *      __virtio16 used_flags;
 181  *      __virtio16 used_idx;
 182  *      struct vring_used_elem used[num];
 183  *      __virtio16 avail_event_idx;
 184  * };
 185  */
 186 /* We publish the used event index at the end of the available ring, and vice
 187  * versa. They are at the end for backwards compatibility. */
 188 #define vring_used_event(vr) ((vr)->avail->ring[(vr)->num])
 189 #define vring_avail_event(vr) (*(__virtio16 *)&(vr)->used->ring[(vr)->num])
 190
 191 static inline void vring_init(struct vring *vr, unsigned int num, void *p,
 192                               unsigned long align)
 193 {
 194         vr->num = num;
 195         vr->desc = p;
 196         vr->avail = (struct vring_avail *)((char *)p + num * sizeof(struct vring_desc));
 197         vr->used = (void *)(((uintptr_t)&vr->avail->ring[num] + sizeof(__virtio16)
 198                 + align-1) & ~(align - 1));
 199 }
 200
 201 static inline unsigned vring_size(unsigned int num, unsigned long align)
 202 {
 203         return ((sizeof(struct vring_desc) * num + sizeof(__virtio16) * (3 + num)
 204                  + align - 1) & ~(align - 1))
 205                 + sizeof(__virtio16) * 3 + sizeof(struct vring_used_elem) * num;
 206 }
 207
 208 #endif /* VIRTIO_RING_NO_LEGACY */
 209
 210 /* The following is used with USED_EVENT_IDX and AVAIL_EVENT_IDX */
 211 /* Assuming a given event_idx value from the other side, if
 212  * we have just incremented index from old to new_idx,
 213  * should we trigger an event? */
 214 static inline int vring_need_event(uint16_t event_idx, uint16_t new_idx, uint16_t old)
 215 {
 216         /* Note: Xen has similar logic for notification hold-off
 217          * in include/xen/interface/io/ring.h with req_event and req_prod
 218          * corresponding to event_idx + 1 and new_idx respectively.
 219          * Note also that req_event and req_prod in Xen start at 1,
 220          * event indexes in virtio start at 0. */
 221         return (uint16_t)(new_idx - event_idx - 1) < (uint16_t)(new_idx - old);
 222 }
 223
 224 struct vring_packed_desc_event {
 225         /* Descriptor Ring Change Event Offset/Wrap Counter. */
 226         uint16_t off_wrap;
 227         /* Descriptor Ring Change Event Flags. */
 228         uint16_t flags;
 229 };
 230
 231 struct vring_packed_desc {
 232         /* Buffer Address. */
 233         uint64_t addr;
 234         /* Buffer Length. */
 235         uint32_t len;
 236         /* Buffer ID. */
 237         uint16_t id;
 238         /* The flags depending on descriptor type. */
 239         uint16_t flags;
 240 };
 241
 242 #endif /* _LINUX_VIRTIO_RING_H */