tools/testing/selftests/kvm/rseq_test.c

Source file repositories/reference/linux-study-clean/tools/testing/selftests/kvm/rseq_test.c

File Facts

System
Linux kernel
Corpus path
tools/testing/selftests/kvm/rseq_test.c
Extension
.c
Size
9972 bytes
Lines
327
Domain
Support Tooling And Documentation
Bucket
tools
Inferred role
Support Tooling And Documentation: implementation source
Status
source implementation candidate

Why This File Exists

Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

Dependency Surface

Detected Declarations

Annotated Snippet

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.

Annotation

Implementation Notes