// 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\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;
}