Add a test to verify an rseq's CPU ID is updated correctly if the task is
migrated while the kernel is handling KVM_RUN. This is a regression test
for a bug introduced by commit 72c3c0fe54a3 ("x86/kvm: Use generic xfer
to guest work function"), where TIF_NOTIFY_RESUME would be cleared by KVM
without updating rseq, leading to a stale CPU ID and other badness.
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
tools/testing/selftests/kvm/.gitignore | 1 +
tools/testing/selftests/kvm/Makefile | 3 +
tools/testing/selftests/kvm/rseq_test.c | 236 ++++++++++++++++++++++++
3 files changed, 240 insertions(+)
create mode 100644 tools/testing/selftests/kvm/rseq_test.c
diff --git a/tools/testing/selftests/kvm/.gitignore b/tools/testing/selftests/kvm/.gitignore
index 0709af0144c8..6d031ff6b68e 100644
--- a/tools/testing/selftests/kvm/.gitignore
+++ b/tools/testing/selftests/kvm/.gitignore
@@ -47,6 +47,7 @@
/kvm_page_table_test
/memslot_modification_stress_test
/memslot_perf_test
+/rseq_test
/set_memory_region_test
/steal_time
/kvm_binary_stats_test
diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
index 5832f510a16c..0756e79cb513 100644
--- a/tools/testing/selftests/kvm/Makefile
+++ b/tools/testing/selftests/kvm/Makefile
@@ -80,6 +80,7 @@ TEST_GEN_PROGS_x86_64 += kvm_create_max_vcpus
TEST_GEN_PROGS_x86_64 += kvm_page_table_test
TEST_GEN_PROGS_x86_64 += memslot_modification_stress_test
TEST_GEN_PROGS_x86_64 += memslot_perf_test
+TEST_GEN_PROGS_x86_64 += rseq_test
TEST_GEN_PROGS_x86_64 += set_memory_region_test
TEST_GEN_PROGS_x86_64 += steal_time
TEST_GEN_PROGS_x86_64 += kvm_binary_stats_test
@@ -92,6 +93,7 @@ TEST_GEN_PROGS_aarch64 += dirty_log_test
TEST_GEN_PROGS_aarch64 += dirty_log_perf_test
TEST_GEN_PROGS_aarch64 += kvm_create_max_vcpus
TEST_GEN_PROGS_aarch64 += kvm_page_table_test
+TEST_GEN_PROGS_aarch64 += rseq_test
TEST_GEN_PROGS_aarch64 += set_memory_region_test
TEST_GEN_PROGS_aarch64 += steal_time
TEST_GEN_PROGS_aarch64 += kvm_binary_stats_test
@@ -103,6 +105,7 @@ TEST_GEN_PROGS_s390x += demand_paging_test
TEST_GEN_PROGS_s390x += dirty_log_test
TEST_GEN_PROGS_s390x += kvm_create_max_vcpus
TEST_GEN_PROGS_s390x += kvm_page_table_test
+TEST_GEN_PROGS_s390x += rseq_test
TEST_GEN_PROGS_s390x += set_memory_region_test
TEST_GEN_PROGS_s390x += kvm_binary_stats_test
diff --git a/tools/testing/selftests/kvm/rseq_test.c b/tools/testing/selftests/kvm/rseq_test.c
new file mode 100644
index 000000000000..060538bd405a
--- /dev/null
+++ b/tools/testing/selftests/kvm/rseq_test.c
@@ -0,0 +1,236 @@
+// 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 <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"
+
+#define VCPU_ID 0
+
+static __thread volatile struct rseq __rseq = {
+ .cpu_id = RSEQ_CPU_ID_UNINITIALIZED,
+};
+
+/*
+ * Use an arbitrary, bogus signature for configuring rseq, this test does not
+ * actually enter an rseq critical section.
+ */
+#define RSEQ_SIG 0xdeadbeef
+
+/*
+ * 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 bool done;
+
+static atomic_t seq_cnt;
+
+static void guest_code(void)
+{
+ for (;;)
+ GUEST_SYNC(0);
+}
+
+static void sys_rseq(int flags)
+{
+ int r;
+
+ r = syscall(__NR_rseq, &__rseq, sizeof(__rseq), flags, RSEQ_SIG);
+ TEST_ASSERT(!r, "rseq failed, errno = %d (%s)", errno, strerror(errno));
+}
+
+static void *migration_worker(void *ign)
+{
+ cpu_set_t allowed_mask;
+ int r, i, nr_cpus, cpu;
+
+ CPU_ZERO(&allowed_mask);
+
+ nr_cpus = CPU_COUNT(&possible_mask);
+
+ for (i = 0; i < NR_TASK_MIGRATIONS; i++) {
+ cpu = i % nr_cpus;
+ if (!CPU_ISSET(cpu, &possible_mask))
+ continue;
+
+ 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 sched_getcpu() isn't
+ * stable, i.e. while changing affinity is in-progress.
+ */
+ smp_wmb();
+ r = sched_setaffinity(0, 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 sched_getcpu() involves a syscall. Stalling the
+ * read-side means the test will spend more time waiting for
+ * sched_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;
+}
+
+int main(int argc, char *argv[])
+{
+ int r, i, snapshot;
+ struct kvm_vm *vm;
+ 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));
+
+ if (CPU_COUNT(&possible_mask) < 2) {
+ print_skip("Only one CPU, task migration not possible\n");
+ exit(KSFT_SKIP);
+ }
+
+ sys_rseq(0);
+
+ /*
+ * 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_default(VCPU_ID, 0, guest_code);
+
+ pthread_create(&migration_thread, NULL, migration_worker, 0);
+
+ for (i = 0; !done; i++) {
+ vcpu_run(vm, VCPU_ID);
+ TEST_ASSERT(get_ucall(vm, VCPU_ID, NULL) == UCALL_SYNC,
+ "Guest failed?");
+
+ /*
+ * Verify rseq's CPU matches sched's CPU. Ensure migration
+ * doesn't occur between sched_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 reading sched_getcpu() and rseq.cpu_id
+ * complete in a single "no migration" window, i.e. are
+ * not reordered across the seq_cnt reads.
+ */
+ smp_rmb();
+ cpu = sched_getcpu();
+ rseq_cpu = READ_ONCE(__rseq.cpu_id);
+ 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
+ * sched_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);
+
+ sys_rseq(RSEQ_FLAG_UNREGISTER);
+
+ return 0;
+}
--
2.33.0.153.gba50c8fa24-goog
