summaryrefslogtreecommitdiffstats
path: root/arch/riscv/kernel/unaligned_access_speed.c
blob: a9a6bcb02acf111cee35a274716a1c78cbdf5886 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
// SPDX-License-Identifier: GPL-2.0-only
/*
 * Copyright 2024 Rivos Inc.
 */

#include <linux/cpu.h>
#include <linux/cpumask.h>
#include <linux/jump_label.h>
#include <linux/mm.h>
#include <linux/smp.h>
#include <linux/types.h>
#include <asm/cpufeature.h>
#include <asm/hwprobe.h>

#include "copy-unaligned.h"

#define MISALIGNED_ACCESS_JIFFIES_LG2 1
#define MISALIGNED_BUFFER_SIZE 0x4000
#define MISALIGNED_BUFFER_ORDER get_order(MISALIGNED_BUFFER_SIZE)
#define MISALIGNED_COPY_SIZE ((MISALIGNED_BUFFER_SIZE / 2) - 0x80)

DEFINE_PER_CPU(long, misaligned_access_speed);

#ifdef CONFIG_RISCV_PROBE_UNALIGNED_ACCESS
static cpumask_t fast_misaligned_access;
static int check_unaligned_access(void *param)
{
	int cpu = smp_processor_id();
	u64 start_cycles, end_cycles;
	u64 word_cycles;
	u64 byte_cycles;
	int ratio;
	unsigned long start_jiffies, now;
	struct page *page = param;
	void *dst;
	void *src;
	long speed = RISCV_HWPROBE_MISALIGNED_SLOW;

	if (per_cpu(misaligned_access_speed, cpu) != RISCV_HWPROBE_MISALIGNED_UNKNOWN)
		return 0;

	/* Make an unaligned destination buffer. */
	dst = (void *)((unsigned long)page_address(page) | 0x1);
	/* Unalign src as well, but differently (off by 1 + 2 = 3). */
	src = dst + (MISALIGNED_BUFFER_SIZE / 2);
	src += 2;
	word_cycles = -1ULL;
	/* Do a warmup. */
	__riscv_copy_words_unaligned(dst, src, MISALIGNED_COPY_SIZE);
	preempt_disable();
	start_jiffies = jiffies;
	while ((now = jiffies) == start_jiffies)
		cpu_relax();

	/*
	 * For a fixed amount of time, repeatedly try the function, and take
	 * the best time in cycles as the measurement.
	 */
	while (time_before(jiffies, now + (1 << MISALIGNED_ACCESS_JIFFIES_LG2))) {
		start_cycles = get_cycles64();
		/* Ensure the CSR read can't reorder WRT to the copy. */
		mb();
		__riscv_copy_words_unaligned(dst, src, MISALIGNED_COPY_SIZE);
		/* Ensure the copy ends before the end time is snapped. */
		mb();
		end_cycles = get_cycles64();
		if ((end_cycles - start_cycles) < word_cycles)
			word_cycles = end_cycles - start_cycles;
	}

	byte_cycles = -1ULL;
	__riscv_copy_bytes_unaligned(dst, src, MISALIGNED_COPY_SIZE);
	start_jiffies = jiffies;
	while ((now = jiffies) == start_jiffies)
		cpu_relax();

	while (time_before(jiffies, now + (1 << MISALIGNED_ACCESS_JIFFIES_LG2))) {
		start_cycles = get_cycles64();
		mb();
		__riscv_copy_bytes_unaligned(dst, src, MISALIGNED_COPY_SIZE);
		mb();
		end_cycles = get_cycles64();
		if ((end_cycles - start_cycles) < byte_cycles)
			byte_cycles = end_cycles - start_cycles;
	}

	preempt_enable();

	/* Don't divide by zero. */
	if (!word_cycles || !byte_cycles) {
		pr_warn("cpu%d: rdtime lacks granularity needed to measure unaligned access speed\n",
			cpu);

		return 0;
	}

	if (word_cycles < byte_cycles)
		speed = RISCV_HWPROBE_MISALIGNED_FAST;

	ratio = div_u64((byte_cycles * 100), word_cycles);
	pr_info("cpu%d: Ratio of byte access time to unaligned word access is %d.%02d, unaligned accesses are %s\n",
		cpu,
		ratio / 100,
		ratio % 100,
		(speed == RISCV_HWPROBE_MISALIGNED_FAST) ? "fast" : "slow");

	per_cpu(misaligned_access_speed, cpu) = speed;

	/*
	 * Set the value of fast_misaligned_access of a CPU. These operations
	 * are atomic to avoid race conditions.
	 */
	if (speed == RISCV_HWPROBE_MISALIGNED_FAST)
		cpumask_set_cpu(cpu, &fast_misaligned_access);
	else
		cpumask_clear_cpu(cpu, &fast_misaligned_access);

	return 0;
}

static void check_unaligned_access_nonboot_cpu(void *param)
{
	unsigned int cpu = smp_processor_id();
	struct page **pages = param;

	if (smp_processor_id() != 0)
		check_unaligned_access(pages[cpu]);
}

DEFINE_STATIC_KEY_FALSE(fast_unaligned_access_speed_key);

static void modify_unaligned_access_branches(cpumask_t *mask, int weight)
{
	if (cpumask_weight(mask) == weight)
		static_branch_enable_cpuslocked(&fast_unaligned_access_speed_key);
	else
		static_branch_disable_cpuslocked(&fast_unaligned_access_speed_key);
}

static void set_unaligned_access_static_branches_except_cpu(int cpu)
{
	/*
	 * Same as set_unaligned_access_static_branches, except excludes the
	 * given CPU from the result. When a CPU is hotplugged into an offline
	 * state, this function is called before the CPU is set to offline in
	 * the cpumask, and thus the CPU needs to be explicitly excluded.
	 */

	cpumask_t fast_except_me;

	cpumask_and(&fast_except_me, &fast_misaligned_access, cpu_online_mask);
	cpumask_clear_cpu(cpu, &fast_except_me);

	modify_unaligned_access_branches(&fast_except_me, num_online_cpus() - 1);
}

static void set_unaligned_access_static_branches(void)
{
	/*
	 * This will be called after check_unaligned_access_all_cpus so the
	 * result of unaligned access speed for all CPUs will be available.
	 *
	 * To avoid the number of online cpus changing between reading
	 * cpu_online_mask and calling num_online_cpus, cpus_read_lock must be
	 * held before calling this function.
	 */

	cpumask_t fast_and_online;

	cpumask_and(&fast_and_online, &fast_misaligned_access, cpu_online_mask);

	modify_unaligned_access_branches(&fast_and_online, num_online_cpus());
}

static int lock_and_set_unaligned_access_static_branch(void)
{
	cpus_read_lock();
	set_unaligned_access_static_branches();
	cpus_read_unlock();

	return 0;
}

arch_initcall_sync(lock_and_set_unaligned_access_static_branch);

static int riscv_online_cpu(unsigned int cpu)
{
	static struct page *buf;

	/* We are already set since the last check */
	if (per_cpu(misaligned_access_speed, cpu) != RISCV_HWPROBE_MISALIGNED_UNKNOWN)
		goto exit;

	buf = alloc_pages(GFP_KERNEL, MISALIGNED_BUFFER_ORDER);
	if (!buf) {
		pr_warn("Allocation failure, not measuring misaligned performance\n");
		return -ENOMEM;
	}

	check_unaligned_access(buf);
	__free_pages(buf, MISALIGNED_BUFFER_ORDER);

exit:
	set_unaligned_access_static_branches();

	return 0;
}

static int riscv_offline_cpu(unsigned int cpu)
{
	set_unaligned_access_static_branches_except_cpu(cpu);

	return 0;
}

/* Measure unaligned access speed on all CPUs present at boot in parallel. */
static int check_unaligned_access_speed_all_cpus(void)
{
	unsigned int cpu;
	unsigned int cpu_count = num_possible_cpus();
	struct page **bufs = kcalloc(cpu_count, sizeof(*bufs), GFP_KERNEL);

	if (!bufs) {
		pr_warn("Allocation failure, not measuring misaligned performance\n");
		return 0;
	}

	/*
	 * Allocate separate buffers for each CPU so there's no fighting over
	 * cache lines.
	 */
	for_each_cpu(cpu, cpu_online_mask) {
		bufs[cpu] = alloc_pages(GFP_KERNEL, MISALIGNED_BUFFER_ORDER);
		if (!bufs[cpu]) {
			pr_warn("Allocation failure, not measuring misaligned performance\n");
			goto out;
		}
	}

	/* Check everybody except 0, who stays behind to tend jiffies. */
	on_each_cpu(check_unaligned_access_nonboot_cpu, bufs, 1);

	/* Check core 0. */
	smp_call_on_cpu(0, check_unaligned_access, bufs[0], true);

	/*
	 * Setup hotplug callbacks for any new CPUs that come online or go
	 * offline.
	 */
	cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, "riscv:online",
				  riscv_online_cpu, riscv_offline_cpu);

out:
	for_each_cpu(cpu, cpu_online_mask) {
		if (bufs[cpu])
			__free_pages(bufs[cpu], MISALIGNED_BUFFER_ORDER);
	}

	kfree(bufs);
	return 0;
}

static int check_unaligned_access_all_cpus(void)
{
	bool all_cpus_emulated = check_unaligned_access_emulated_all_cpus();

	if (!all_cpus_emulated)
		return check_unaligned_access_speed_all_cpus();

	return 0;
}
#else /* CONFIG_RISCV_PROBE_UNALIGNED_ACCESS */
static int check_unaligned_access_all_cpus(void)
{
	check_unaligned_access_emulated_all_cpus();

	return 0;
}
#endif

arch_initcall(check_unaligned_access_all_cpus);