blob: 4158da0da2bba8e5e8ea12acd87f6cff6206e3ae [file] [log] [blame]
// 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"
#define VCPU_ID 0
static __thread volatile struct rseq __rseq = {
* 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 int min_cpu, max_cpu;
static bool done;
static atomic_t seq_cnt;
static void guest_code(void)
for (;;)
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 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 {
if (cpu > max_cpu) {
cpu = min_cpu;
TEST_ASSERT(CPU_ISSET(cpu, &possible_mask),
"Min CPU = %d must always be usable", cpu);
} while (!CPU_ISSET(cpu, &possible_mask));
return cpu;
static void *migration_worker(void *ign)
cpu_set_t allowed_mask;
int r, i, cpu;
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.
* Ensure the odd count is visible while sched_getcpu() isn't
* stable, i.e. while changing affinity is in-progress.
r = sched_setaffinity(0, sizeof(allowed_mask), &allowed_mask);
TEST_ASSERT(!r, "sched_setaffinity failed, errno = %d (%s)",
errno, strerror(errno));
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;
static int calc_min_max_cpu(void)
int i, cnt, nproc;
if (CPU_COUNT(&possible_mask) < 2)
return -EINVAL;
* 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))
if (min_cpu == -1)
min_cpu = i;
max_cpu = i;
return (cnt < 2) ? -EINVAL : 0;
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,
if (calc_min_max_cpu()) {
print_skip("Only one usable CPU, task migration not possible");
* 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);
ucall_init(vm, NULL);
pthread_create(&migration_thread, NULL, migration_worker, 0);
for (i = 0; !done; i++) {
vcpu_run(vm, VCPU_ID);
"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.
cpu = sched_getcpu();
rseq_cpu = READ_ONCE(__rseq.cpu_id);
} 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.
"Only performed %d KVM_RUNs, task stalled too much?\n", i);
pthread_join(migration_thread, NULL);
return 0;