1 // SPDX-License-Identifier: GPL-2.0-only
2 #define _GNU_SOURCE /* for program_invocation_short_name */
3 #include <errno.h>
4 #include <fcntl.h>
5 #include <pthread.h>
6 #include <sched.h>
7 #include <stdio.h>
8 #include <stdlib.h>
9 #include <string.h>
10 #include <signal.h>
11 #include <syscall.h>
12 #include <sys/ioctl.h>
13 #include <sys/sysinfo.h>
14 #include <asm/barrier.h>
15 #include <linux/atomic.h>
16 #include <linux/rseq.h>
17 #include <linux/unistd.h>
18
19 #include "kvm_util.h"
20 #include "processor.h"
21 #include "test_util.h"
22
23 #include "../rseq/rseq.c"
24
25 /*
26 * Any bug related to task migration is likely to be timing-dependent; perform
27 * a large number of migrations to reduce the odds of a false negative.
28 */
29 #define NR_TASK_MIGRATIONS 100000
30
31 static pthread_t migration_thread;
32 static cpu_set_t possible_mask;
33 static int min_cpu, max_cpu;
34 static bool done;
35
36 static atomic_t seq_cnt;
37
guest_code(void)38 static void guest_code(void)
39 {
40 for (;;)
41 GUEST_SYNC(0);
42 }
43
44 /*
45 * We have to perform direct system call for getcpu() because it's
46 * not available until glic 2.29.
47 */
sys_getcpu(unsigned * cpu)48 static void sys_getcpu(unsigned *cpu)
49 {
50 int r;
51
52 r = syscall(__NR_getcpu, cpu, NULL, NULL);
53 TEST_ASSERT(!r, "getcpu failed, errno = %d (%s)", errno, strerror(errno));
54 }
55
next_cpu(int cpu)56 static int next_cpu(int cpu)
57 {
58 /*
59 * Advance to the next CPU, skipping those that weren't in the original
60 * affinity set. Sadly, there is no CPU_SET_FOR_EACH, and cpu_set_t's
61 * data storage is considered as opaque. Note, if this task is pinned
62 * to a small set of discontigous CPUs, e.g. 2 and 1023, this loop will
63 * burn a lot cycles and the test will take longer than normal to
64 * complete.
65 */
66 do {
67 cpu++;
68 if (cpu > max_cpu) {
69 cpu = min_cpu;
70 TEST_ASSERT(CPU_ISSET(cpu, &possible_mask),
71 "Min CPU = %d must always be usable", cpu);
72 break;
73 }
74 } while (!CPU_ISSET(cpu, &possible_mask));
75
76 return cpu;
77 }
78
migration_worker(void * __rseq_tid)79 static void *migration_worker(void *__rseq_tid)
80 {
81 pid_t rseq_tid = (pid_t)(unsigned long)__rseq_tid;
82 cpu_set_t allowed_mask;
83 int r, i, cpu;
84
85 CPU_ZERO(&allowed_mask);
86
87 for (i = 0, cpu = min_cpu; i < NR_TASK_MIGRATIONS; i++, cpu = next_cpu(cpu)) {
88 CPU_SET(cpu, &allowed_mask);
89
90 /*
91 * Bump the sequence count twice to allow the reader to detect
92 * that a migration may have occurred in between rseq and sched
93 * CPU ID reads. An odd sequence count indicates a migration
94 * is in-progress, while a completely different count indicates
95 * a migration occurred since the count was last read.
96 */
97 atomic_inc(&seq_cnt);
98
99 /*
100 * Ensure the odd count is visible while getcpu() isn't
101 * stable, i.e. while changing affinity is in-progress.
102 */
103 smp_wmb();
104 r = sched_setaffinity(rseq_tid, sizeof(allowed_mask), &allowed_mask);
105 TEST_ASSERT(!r, "sched_setaffinity failed, errno = %d (%s)",
106 errno, strerror(errno));
107 smp_wmb();
108 atomic_inc(&seq_cnt);
109
110 CPU_CLR(cpu, &allowed_mask);
111
112 /*
113 * Wait 1-10us before proceeding to the next iteration and more
114 * specifically, before bumping seq_cnt again. A delay is
115 * needed on three fronts:
116 *
117 * 1. To allow sched_setaffinity() to prompt migration before
118 * ioctl(KVM_RUN) enters the guest so that TIF_NOTIFY_RESUME
119 * (or TIF_NEED_RESCHED, which indirectly leads to handling
120 * NOTIFY_RESUME) is handled in KVM context.
121 *
122 * If NOTIFY_RESUME/NEED_RESCHED is set after KVM enters
123 * the guest, the guest will trigger a IO/MMIO exit all the
124 * way to userspace and the TIF flags will be handled by
125 * the generic "exit to userspace" logic, not by KVM. The
126 * exit to userspace is necessary to give the test a chance
127 * to check the rseq CPU ID (see #2).
128 *
129 * Alternatively, guest_code() could include an instruction
130 * to trigger an exit that is handled by KVM, but any such
131 * exit requires architecture specific code.
132 *
133 * 2. To let ioctl(KVM_RUN) make its way back to the test
134 * before the next round of migration. The test's check on
135 * the rseq CPU ID must wait for migration to complete in
136 * order to avoid false positive, thus any kernel rseq bug
137 * will be missed if the next migration starts before the
138 * check completes.
139 *
140 * 3. To ensure the read-side makes efficient forward progress,
141 * e.g. if getcpu() involves a syscall. Stalling the read-side
142 * means the test will spend more time waiting for getcpu()
143 * to stabilize and less time trying to hit the timing-dependent
144 * bug.
145 *
146 * Because any bug in this area is likely to be timing-dependent,
147 * run with a range of delays at 1us intervals from 1us to 10us
148 * as a best effort to avoid tuning the test to the point where
149 * it can hit _only_ the original bug and not detect future
150 * regressions.
151 *
152 * The original bug can reproduce with a delay up to ~500us on
153 * x86-64, but starts to require more iterations to reproduce
154 * as the delay creeps above ~10us, and the average runtime of
155 * each iteration obviously increases as well. Cap the delay
156 * at 10us to keep test runtime reasonable while minimizing
157 * potential coverage loss.
158 *
159 * The lower bound for reproducing the bug is likely below 1us,
160 * e.g. failures occur on x86-64 with nanosleep(0), but at that
161 * point the overhead of the syscall likely dominates the delay.
162 * Use usleep() for simplicity and to avoid unnecessary kernel
163 * dependencies.
164 */
165 usleep((i % 10) + 1);
166 }
167 done = true;
168 return NULL;
169 }
170
calc_min_max_cpu(void)171 static void calc_min_max_cpu(void)
172 {
173 int i, cnt, nproc;
174
175 TEST_REQUIRE(CPU_COUNT(&possible_mask) >= 2);
176
177 /*
178 * CPU_SET doesn't provide a FOR_EACH helper, get the min/max CPU that
179 * this task is affined to in order to reduce the time spent querying
180 * unusable CPUs, e.g. if this task is pinned to a small percentage of
181 * total CPUs.
182 */
183 nproc = get_nprocs_conf();
184 min_cpu = -1;
185 max_cpu = -1;
186 cnt = 0;
187
188 for (i = 0; i < nproc; i++) {
189 if (!CPU_ISSET(i, &possible_mask))
190 continue;
191 if (min_cpu == -1)
192 min_cpu = i;
193 max_cpu = i;
194 cnt++;
195 }
196
197 __TEST_REQUIRE(cnt >= 2,
198 "Only one usable CPU, task migration not possible");
199 }
200
main(int argc,char * argv[])201 int main(int argc, char *argv[])
202 {
203 int r, i, snapshot;
204 struct kvm_vm *vm;
205 struct kvm_vcpu *vcpu;
206 u32 cpu, rseq_cpu;
207
208 /* Tell stdout not to buffer its content */
209 setbuf(stdout, NULL);
210
211 r = sched_getaffinity(0, sizeof(possible_mask), &possible_mask);
212 TEST_ASSERT(!r, "sched_getaffinity failed, errno = %d (%s)", errno,
213 strerror(errno));
214
215 calc_min_max_cpu();
216
217 r = rseq_register_current_thread();
218 TEST_ASSERT(!r, "rseq_register_current_thread failed, errno = %d (%s)",
219 errno, strerror(errno));
220
221 /*
222 * Create and run a dummy VM that immediately exits to userspace via
223 * GUEST_SYNC, while concurrently migrating the process by setting its
224 * CPU affinity.
225 */
226 vm = vm_create_with_one_vcpu(&vcpu, guest_code);
227 ucall_init(vm, NULL);
228
229 pthread_create(&migration_thread, NULL, migration_worker,
230 (void *)(unsigned long)syscall(SYS_gettid));
231
232 for (i = 0; !done; i++) {
233 vcpu_run(vcpu);
234 TEST_ASSERT(get_ucall(vcpu, NULL) == UCALL_SYNC,
235 "Guest failed?");
236
237 /*
238 * Verify rseq's CPU matches sched's CPU. Ensure migration
239 * doesn't occur between getcpu() and reading the rseq cpu_id
240 * by rereading both if the sequence count changes, or if the
241 * count is odd (migration in-progress).
242 */
243 do {
244 /*
245 * Drop bit 0 to force a mismatch if the count is odd,
246 * i.e. if a migration is in-progress.
247 */
248 snapshot = atomic_read(&seq_cnt) & ~1;
249
250 /*
251 * Ensure calling getcpu() and reading rseq.cpu_id complete
252 * in a single "no migration" window, i.e. are not reordered
253 * across the seq_cnt reads.
254 */
255 smp_rmb();
256 sys_getcpu(&cpu);
257 rseq_cpu = rseq_current_cpu_raw();
258 smp_rmb();
259 } while (snapshot != atomic_read(&seq_cnt));
260
261 TEST_ASSERT(rseq_cpu == cpu,
262 "rseq CPU = %d, sched CPU = %d\n", rseq_cpu, cpu);
263 }
264
265 /*
266 * Sanity check that the test was able to enter the guest a reasonable
267 * number of times, e.g. didn't get stalled too often/long waiting for
268 * getcpu() to stabilize. A 2:1 migration:KVM_RUN ratio is a fairly
269 * conservative ratio on x86-64, which can do _more_ KVM_RUNs than
270 * migrations given the 1us+ delay in the migration task.
271 */
272 TEST_ASSERT(i > (NR_TASK_MIGRATIONS / 2),
273 "Only performed %d KVM_RUNs, task stalled too much?\n", i);
274
275 pthread_join(migration_thread, NULL);
276
277 kvm_vm_free(vm);
278
279 rseq_unregister_current_thread();
280
281 return 0;
282 }
283