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