1 files changed, 268 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 000000000000..28f97fb52044
--- /dev/null
+++ b/tools/testing/selftests/kvm/rseq_test.c
@@ -0,0 +1,268 @@
+// 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);
+}
+
+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;
+
+	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);
+
+	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();
+			r = sys_getcpu(&cpu, NULL);
+			TEST_ASSERT(!r, "getcpu failed, errno = %d (%s)",
+				    errno, strerror(errno));
+			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", 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?", i);
+
+	pthread_join(migration_thread, NULL);
+
+	kvm_vm_free(vm);
+
+	rseq_unregister_current_thread();
+
+	return 0;
+}