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-rw-r--r--tools/testing/selftests/kvm/rseq_test.c282
1 files changed, 282 insertions, 0 deletions
diff --git a/tools/testing/selftests/kvm/rseq_test.c b/tools/testing/selftests/kvm/rseq_test.c
new file mode 100644
index 000000000..6f88da7e6
--- /dev/null
+++ b/tools/testing/selftests/kvm/rseq_test.c
@@ -0,0 +1,282 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#define _GNU_SOURCE /* for program_invocation_short_name */
+#include <errno.h>
+#include <fcntl.h>
+#include <pthread.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <signal.h>
+#include <syscall.h>
+#include <sys/ioctl.h>
+#include <sys/sysinfo.h>
+#include <asm/barrier.h>
+#include <linux/atomic.h>
+#include <linux/rseq.h>
+#include <linux/unistd.h>
+
+#include "kvm_util.h"
+#include "processor.h"
+#include "test_util.h"
+
+#include "../rseq/rseq.c"
+
+/*
+ * Any bug related to task migration is likely to be timing-dependent; perform
+ * a large number of migrations to reduce the odds of a false negative.
+ */
+#define NR_TASK_MIGRATIONS 100000
+
+static pthread_t migration_thread;
+static cpu_set_t possible_mask;
+static int min_cpu, max_cpu;
+static bool done;
+
+static atomic_t seq_cnt;
+
+static void guest_code(void)
+{
+ for (;;)
+ GUEST_SYNC(0);
+}
+
+/*
+ * We have to perform direct system call for getcpu() because it's
+ * not available until glic 2.29.
+ */
+static void sys_getcpu(unsigned *cpu)
+{
+ int r;
+
+ r = syscall(__NR_getcpu, cpu, NULL, NULL);
+ TEST_ASSERT(!r, "getcpu failed, errno = %d (%s)", errno, strerror(errno));
+}
+
+static int next_cpu(int cpu)
+{
+ /*
+ * Advance to the next CPU, skipping those that weren't in the original
+ * affinity set. Sadly, there is no CPU_SET_FOR_EACH, and cpu_set_t's
+ * data storage is considered as opaque. Note, if this task is pinned
+ * to a small set of discontigous CPUs, e.g. 2 and 1023, this loop will
+ * burn a lot cycles and the test will take longer than normal to
+ * complete.
+ */
+ do {
+ cpu++;
+ if (cpu > max_cpu) {
+ cpu = min_cpu;
+ TEST_ASSERT(CPU_ISSET(cpu, &possible_mask),
+ "Min CPU = %d must always be usable", cpu);
+ break;
+ }
+ } while (!CPU_ISSET(cpu, &possible_mask));
+
+ return cpu;
+}
+
+static void *migration_worker(void *__rseq_tid)
+{
+ pid_t rseq_tid = (pid_t)(unsigned long)__rseq_tid;
+ cpu_set_t allowed_mask;
+ int r, i, cpu;
+
+ CPU_ZERO(&allowed_mask);
+
+ for (i = 0, cpu = min_cpu; i < NR_TASK_MIGRATIONS; i++, cpu = next_cpu(cpu)) {
+ CPU_SET(cpu, &allowed_mask);
+
+ /*
+ * Bump the sequence count twice to allow the reader to detect
+ * that a migration may have occurred in between rseq and sched
+ * CPU ID reads. An odd sequence count indicates a migration
+ * is in-progress, while a completely different count indicates
+ * a migration occurred since the count was last read.
+ */
+ atomic_inc(&seq_cnt);
+
+ /*
+ * Ensure the odd count is visible while getcpu() isn't
+ * stable, i.e. while changing affinity is in-progress.
+ */
+ smp_wmb();
+ r = sched_setaffinity(rseq_tid, sizeof(allowed_mask), &allowed_mask);
+ TEST_ASSERT(!r, "sched_setaffinity failed, errno = %d (%s)",
+ errno, strerror(errno));
+ smp_wmb();
+ atomic_inc(&seq_cnt);
+
+ CPU_CLR(cpu, &allowed_mask);
+
+ /*
+ * Wait 1-10us before proceeding to the next iteration and more
+ * specifically, before bumping seq_cnt again. A delay is
+ * needed on three fronts:
+ *
+ * 1. To allow sched_setaffinity() to prompt migration before
+ * ioctl(KVM_RUN) enters the guest so that TIF_NOTIFY_RESUME
+ * (or TIF_NEED_RESCHED, which indirectly leads to handling
+ * NOTIFY_RESUME) is handled in KVM context.
+ *
+ * If NOTIFY_RESUME/NEED_RESCHED is set after KVM enters
+ * the guest, the guest will trigger a IO/MMIO exit all the
+ * way to userspace and the TIF flags will be handled by
+ * the generic "exit to userspace" logic, not by KVM. The
+ * exit to userspace is necessary to give the test a chance
+ * to check the rseq CPU ID (see #2).
+ *
+ * Alternatively, guest_code() could include an instruction
+ * to trigger an exit that is handled by KVM, but any such
+ * exit requires architecture specific code.
+ *
+ * 2. To let ioctl(KVM_RUN) make its way back to the test
+ * before the next round of migration. The test's check on
+ * the rseq CPU ID must wait for migration to complete in
+ * order to avoid false positive, thus any kernel rseq bug
+ * will be missed if the next migration starts before the
+ * check completes.
+ *
+ * 3. To ensure the read-side makes efficient forward progress,
+ * e.g. if getcpu() involves a syscall. Stalling the read-side
+ * means the test will spend more time waiting for getcpu()
+ * to stabilize and less time trying to hit the timing-dependent
+ * bug.
+ *
+ * Because any bug in this area is likely to be timing-dependent,
+ * run with a range of delays at 1us intervals from 1us to 10us
+ * as a best effort to avoid tuning the test to the point where
+ * it can hit _only_ the original bug and not detect future
+ * regressions.
+ *
+ * The original bug can reproduce with a delay up to ~500us on
+ * x86-64, but starts to require more iterations to reproduce
+ * as the delay creeps above ~10us, and the average runtime of
+ * each iteration obviously increases as well. Cap the delay
+ * at 10us to keep test runtime reasonable while minimizing
+ * potential coverage loss.
+ *
+ * The lower bound for reproducing the bug is likely below 1us,
+ * e.g. failures occur on x86-64 with nanosleep(0), but at that
+ * point the overhead of the syscall likely dominates the delay.
+ * Use usleep() for simplicity and to avoid unnecessary kernel
+ * dependencies.
+ */
+ usleep((i % 10) + 1);
+ }
+ done = true;
+ return NULL;
+}
+
+static void calc_min_max_cpu(void)
+{
+ int i, cnt, nproc;
+
+ TEST_REQUIRE(CPU_COUNT(&possible_mask) >= 2);
+
+ /*
+ * CPU_SET doesn't provide a FOR_EACH helper, get the min/max CPU that
+ * this task is affined to in order to reduce the time spent querying
+ * unusable CPUs, e.g. if this task is pinned to a small percentage of
+ * total CPUs.
+ */
+ nproc = get_nprocs_conf();
+ min_cpu = -1;
+ max_cpu = -1;
+ cnt = 0;
+
+ for (i = 0; i < nproc; i++) {
+ if (!CPU_ISSET(i, &possible_mask))
+ continue;
+ if (min_cpu == -1)
+ min_cpu = i;
+ max_cpu = i;
+ cnt++;
+ }
+
+ __TEST_REQUIRE(cnt >= 2,
+ "Only one usable CPU, task migration not possible");
+}
+
+int main(int argc, char *argv[])
+{
+ int r, i, snapshot;
+ struct kvm_vm *vm;
+ struct kvm_vcpu *vcpu;
+ u32 cpu, rseq_cpu;
+
+ /* Tell stdout not to buffer its content */
+ setbuf(stdout, NULL);
+
+ r = sched_getaffinity(0, sizeof(possible_mask), &possible_mask);
+ TEST_ASSERT(!r, "sched_getaffinity failed, errno = %d (%s)", errno,
+ strerror(errno));
+
+ calc_min_max_cpu();
+
+ r = rseq_register_current_thread();
+ TEST_ASSERT(!r, "rseq_register_current_thread failed, errno = %d (%s)",
+ errno, strerror(errno));
+
+ /*
+ * Create and run a dummy VM that immediately exits to userspace via
+ * GUEST_SYNC, while concurrently migrating the process by setting its
+ * CPU affinity.
+ */
+ vm = vm_create_with_one_vcpu(&vcpu, guest_code);
+ ucall_init(vm, NULL);
+
+ pthread_create(&migration_thread, NULL, migration_worker,
+ (void *)(unsigned long)syscall(SYS_gettid));
+
+ for (i = 0; !done; i++) {
+ vcpu_run(vcpu);
+ TEST_ASSERT(get_ucall(vcpu, NULL) == UCALL_SYNC,
+ "Guest failed?");
+
+ /*
+ * Verify rseq's CPU matches sched's CPU. Ensure migration
+ * doesn't occur between getcpu() and reading the rseq cpu_id
+ * by rereading both if the sequence count changes, or if the
+ * count is odd (migration in-progress).
+ */
+ do {
+ /*
+ * Drop bit 0 to force a mismatch if the count is odd,
+ * i.e. if a migration is in-progress.
+ */
+ snapshot = atomic_read(&seq_cnt) & ~1;
+
+ /*
+ * Ensure calling getcpu() and reading rseq.cpu_id complete
+ * in a single "no migration" window, i.e. are not reordered
+ * across the seq_cnt reads.
+ */
+ smp_rmb();
+ sys_getcpu(&cpu);
+ rseq_cpu = rseq_current_cpu_raw();
+ smp_rmb();
+ } while (snapshot != atomic_read(&seq_cnt));
+
+ TEST_ASSERT(rseq_cpu == cpu,
+ "rseq CPU = %d, sched CPU = %d\n", rseq_cpu, cpu);
+ }
+
+ /*
+ * Sanity check that the test was able to enter the guest a reasonable
+ * number of times, e.g. didn't get stalled too often/long waiting for
+ * getcpu() to stabilize. A 2:1 migration:KVM_RUN ratio is a fairly
+ * conservative ratio on x86-64, which can do _more_ KVM_RUNs than
+ * migrations given the 1us+ delay in the migration task.
+ */
+ TEST_ASSERT(i > (NR_TASK_MIGRATIONS / 2),
+ "Only performed %d KVM_RUNs, task stalled too much?\n", i);
+
+ pthread_join(migration_thread, NULL);
+
+ kvm_vm_free(vm);
+
+ rseq_unregister_current_thread();
+
+ return 0;
+}