1 /*
2  * Copyright (C) 2016 Red Hat, Inc.
3  * Author: Michael S. Tsirkin <mst@redhat.com>
4  * This work is licensed under the terms of the GNU GPL, version 2.
5  *
6  * Partial implementation of virtio 0.9. event index is used for signalling,
7  * unconditionally. Design roughly follows linux kernel implementation in order
8  * to be able to judge its performance.
9  */
10 #define _GNU_SOURCE
11 #include "main.h"
12 #include <stdlib.h>
13 #include <stdio.h>
14 #include <assert.h>
15 #include <string.h>
16 #include <linux/virtio_ring.h>
17 
18 struct data {
19 	void *data;
20 } *data;
21 
22 struct vring ring;
23 
24 /* enabling the below activates experimental ring polling code
25  * (which skips index reads on consumer in favor of looking at
26  * high bits of ring id ^ 0x8000).
27  */
28 /* #ifdef RING_POLL */
29 /* enabling the below activates experimental in-order code
30  * (which skips ring updates and reads and writes len in descriptor).
31  */
32 /* #ifdef INORDER */
33 
34 #if defined(RING_POLL) && defined(INORDER)
35 #error "RING_POLL and INORDER are mutually exclusive"
36 #endif
37 
38 /* how much padding is needed to avoid false cache sharing */
39 #define HOST_GUEST_PADDING 0x80
40 
41 struct guest {
42 	unsigned short avail_idx;
43 	unsigned short last_used_idx;
44 	unsigned short num_free;
45 	unsigned short kicked_avail_idx;
46 #ifndef INORDER
47 	unsigned short free_head;
48 #else
49 	unsigned short reserved_free_head;
50 #endif
51 	unsigned char reserved[HOST_GUEST_PADDING - 10];
52 } guest;
53 
54 struct host {
55 	/* we do not need to track last avail index
56 	 * unless we have more than one in flight.
57 	 */
58 	unsigned short used_idx;
59 	unsigned short called_used_idx;
60 	unsigned char reserved[HOST_GUEST_PADDING - 4];
61 } host;
62 
63 /* implemented by ring */
64 void alloc_ring(void)
65 {
66 	int ret;
67 	int i;
68 	void *p;
69 
70 	ret = posix_memalign(&p, 0x1000, vring_size(ring_size, 0x1000));
71 	if (ret) {
72 		perror("Unable to allocate ring buffer.\n");
73 		exit(3);
74 	}
75 	memset(p, 0, vring_size(ring_size, 0x1000));
76 	vring_init(&ring, ring_size, p, 0x1000);
77 
78 	guest.avail_idx = 0;
79 	guest.kicked_avail_idx = -1;
80 	guest.last_used_idx = 0;
81 #ifndef INORDER
82 	/* Put everything in free lists. */
83 	guest.free_head = 0;
84 #endif
85 	for (i = 0; i < ring_size - 1; i++)
86 		ring.desc[i].next = i + 1;
87 	host.used_idx = 0;
88 	host.called_used_idx = -1;
89 	guest.num_free = ring_size;
90 	data = malloc(ring_size * sizeof *data);
91 	if (!data) {
92 		perror("Unable to allocate data buffer.\n");
93 		exit(3);
94 	}
95 	memset(data, 0, ring_size * sizeof *data);
96 }
97 
98 /* guest side */
99 int add_inbuf(unsigned len, void *buf, void *datap)
100 {
101 	unsigned head;
102 #ifndef INORDER
103 	unsigned avail;
104 #endif
105 	struct vring_desc *desc;
106 
107 	if (!guest.num_free)
108 		return -1;
109 
110 #ifdef INORDER
111 	head = (ring_size - 1) & (guest.avail_idx++);
112 #else
113 	head = guest.free_head;
114 #endif
115 	guest.num_free--;
116 
117 	desc = ring.desc;
118 	desc[head].flags = VRING_DESC_F_NEXT;
119 	desc[head].addr = (unsigned long)(void *)buf;
120 	desc[head].len = len;
121 	/* We do it like this to simulate the way
122 	 * we'd have to flip it if we had multiple
123 	 * descriptors.
124 	 */
125 	desc[head].flags &= ~VRING_DESC_F_NEXT;
126 #ifndef INORDER
127 	guest.free_head = desc[head].next;
128 #endif
129 
130 	data[head].data = datap;
131 
132 #ifdef RING_POLL
133 	/* Barrier A (for pairing) */
134 	smp_release();
135 	avail = guest.avail_idx++;
136 	ring.avail->ring[avail & (ring_size - 1)] =
137 		(head | (avail & ~(ring_size - 1))) ^ 0x8000;
138 #else
139 #ifndef INORDER
140 	/* Barrier A (for pairing) */
141 	smp_release();
142 	avail = (ring_size - 1) & (guest.avail_idx++);
143 	ring.avail->ring[avail] = head;
144 #endif
145 	/* Barrier A (for pairing) */
146 	smp_release();
147 #endif
148 	ring.avail->idx = guest.avail_idx;
149 	return 0;
150 }
151 
152 void *get_buf(unsigned *lenp, void **bufp)
153 {
154 	unsigned head;
155 	unsigned index;
156 	void *datap;
157 
158 #ifdef RING_POLL
159 	head = (ring_size - 1) & guest.last_used_idx;
160 	index = ring.used->ring[head].id;
161 	if ((index ^ guest.last_used_idx ^ 0x8000) & ~(ring_size - 1))
162 		return NULL;
163 	/* Barrier B (for pairing) */
164 	smp_acquire();
165 	index &= ring_size - 1;
166 #else
167 	if (ring.used->idx == guest.last_used_idx)
168 		return NULL;
169 	/* Barrier B (for pairing) */
170 	smp_acquire();
171 #ifdef INORDER
172 	head = (ring_size - 1) & guest.last_used_idx;
173 	index = head;
174 #else
175 	head = (ring_size - 1) & guest.last_used_idx;
176 	index = ring.used->ring[head].id;
177 #endif
178 
179 #endif
180 #ifdef INORDER
181 	*lenp = ring.desc[index].len;
182 #else
183 	*lenp = ring.used->ring[head].len;
184 #endif
185 	datap = data[index].data;
186 	*bufp = (void*)(unsigned long)ring.desc[index].addr;
187 	data[index].data = NULL;
188 #ifndef INORDER
189 	ring.desc[index].next = guest.free_head;
190 	guest.free_head = index;
191 #endif
192 	guest.num_free++;
193 	guest.last_used_idx++;
194 	return datap;
195 }
196 
197 bool used_empty()
198 {
199 	unsigned short last_used_idx = guest.last_used_idx;
200 #ifdef RING_POLL
201 	unsigned short head = last_used_idx & (ring_size - 1);
202 	unsigned index = ring.used->ring[head].id;
203 
204 	return (index ^ last_used_idx ^ 0x8000) & ~(ring_size - 1);
205 #else
206 	return ring.used->idx == last_used_idx;
207 #endif
208 }
209 
210 void disable_call()
211 {
212 	/* Doing nothing to disable calls might cause
213 	 * extra interrupts, but reduces the number of cache misses.
214 	 */
215 }
216 
217 bool enable_call()
218 {
219 	vring_used_event(&ring) = guest.last_used_idx;
220 	/* Flush call index write */
221 	/* Barrier D (for pairing) */
222 	smp_mb();
223 	return used_empty();
224 }
225 
226 void kick_available(void)
227 {
228 	bool need;
229 
230 	/* Flush in previous flags write */
231 	/* Barrier C (for pairing) */
232 	smp_mb();
233 	need = vring_need_event(vring_avail_event(&ring),
234 				guest.avail_idx,
235 				guest.kicked_avail_idx);
236 
237 	guest.kicked_avail_idx = guest.avail_idx;
238 	if (need)
239 		kick();
240 }
241 
242 /* host side */
243 void disable_kick()
244 {
245 	/* Doing nothing to disable kicks might cause
246 	 * extra interrupts, but reduces the number of cache misses.
247 	 */
248 }
249 
250 bool enable_kick()
251 {
252 	vring_avail_event(&ring) = host.used_idx;
253 	/* Barrier C (for pairing) */
254 	smp_mb();
255 	return avail_empty();
256 }
257 
258 bool avail_empty()
259 {
260 	unsigned head = host.used_idx;
261 #ifdef RING_POLL
262 	unsigned index = ring.avail->ring[head & (ring_size - 1)];
263 
264 	return ((index ^ head ^ 0x8000) & ~(ring_size - 1));
265 #else
266 	return head == ring.avail->idx;
267 #endif
268 }
269 
270 bool use_buf(unsigned *lenp, void **bufp)
271 {
272 	unsigned used_idx = host.used_idx;
273 	struct vring_desc *desc;
274 	unsigned head;
275 
276 #ifdef RING_POLL
277 	head = ring.avail->ring[used_idx & (ring_size - 1)];
278 	if ((used_idx ^ head ^ 0x8000) & ~(ring_size - 1))
279 		return false;
280 	/* Barrier A (for pairing) */
281 	smp_acquire();
282 
283 	used_idx &= ring_size - 1;
284 	desc = &ring.desc[head & (ring_size - 1)];
285 #else
286 	if (used_idx == ring.avail->idx)
287 		return false;
288 
289 	/* Barrier A (for pairing) */
290 	smp_acquire();
291 
292 	used_idx &= ring_size - 1;
293 #ifdef INORDER
294 	head = used_idx;
295 #else
296 	head = ring.avail->ring[used_idx];
297 #endif
298 	desc = &ring.desc[head];
299 #endif
300 
301 	*lenp = desc->len;
302 	*bufp = (void *)(unsigned long)desc->addr;
303 
304 #ifdef INORDER
305 	desc->len = desc->len - 1;
306 #else
307 	/* now update used ring */
308 	ring.used->ring[used_idx].id = head;
309 	ring.used->ring[used_idx].len = desc->len - 1;
310 #endif
311 	/* Barrier B (for pairing) */
312 	smp_release();
313 	host.used_idx++;
314 	ring.used->idx = host.used_idx;
315 
316 	return true;
317 }
318 
319 void call_used(void)
320 {
321 	bool need;
322 
323 	/* Flush in previous flags write */
324 	/* Barrier D (for pairing) */
325 	smp_mb();
326 	need = vring_need_event(vring_used_event(&ring),
327 				host.used_idx,
328 				host.called_used_idx);
329 
330 	host.called_used_idx = host.used_idx;
331 	if (need)
332 		call();
333 }
334