[RFC 1/1] KVM: selftests: rseq_test: use vdso_getcpu() instead of syscall()

[Robert Hoo <robert.hu@xxxxxxxxxxxxxxx>]

vDSO getcpu() has been in Kernel since 2.6.19, which we can assume
generally available.
Use vDSO getcpu() to reduce the overhead, so that vcpu thread stalls less
therefore can have more odds to hit the race condition.

Fixes: 0fcc102923de ("KVM: selftests: Use getcpu() instead of sched_getcpu() in rseq_test")
Signed-off-by: Robert Hoo <robert.hu@xxxxxxxxxxxxxxx>
---
 tools/testing/selftests/kvm/rseq_test.c | 32 ++++++++++++++++++-------
 1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/tools/testing/selftests/kvm/rseq_test.c b/tools/testing/selftests/kvm/rseq_test.c
index 6f88da7e60be..0b68a6b19b31 100644
--- a/tools/testing/selftests/kvm/rseq_test.c
+++ b/tools/testing/selftests/kvm/rseq_test.c
@@ -42,15 +42,29 @@ static void guest_code(void)
 }
 
 /*
- * We have to perform direct system call for getcpu() because it's
- * not available until glic 2.29.
+ * getcpu() was added in kernel 2.6.19. glibc support wasn't there
+ * until glibc 2.29.
+ * We can direct call it from vdso to ease gblic dependency.
+ *
+ * vdso manipulation code refers from selftests/x86/test_vsyscall.c
  */
-static void sys_getcpu(unsigned *cpu)
-{
-	int r;
+typedef long (*getcpu_t)(unsigned *, unsigned *, void *);
+static getcpu_t vdso_getcpu;
 
-	r = syscall(__NR_getcpu, cpu, NULL, NULL);
-	TEST_ASSERT(!r, "getcpu failed, errno = %d (%s)", errno, strerror(errno));
+static void init_vdso(void)
+{
+	void *vdso = dlopen("linux-vdso.so.1", RTLD_LAZY | RTLD_LOCAL |
+			    RTLD_NOLOAD);
+	if (!vdso)
+		vdso = dlopen("linux-gate.so.1", RTLD_LAZY | RTLD_LOCAL |
+			      RTLD_NOLOAD);
+	if (!vdso)
+		TEST_ASSERT(!vdso, "failed to find vDSO\n");
+
+	vdso_getcpu = (getcpu_t)dlsym(vdso, "__vdso_getcpu");
+	if (!vdso_getcpu)
+		TEST_ASSERT(!vdso_getcpu,
+			    "failed to find __vdso_getcpu in vDSO\n");
 }
 
 static int next_cpu(int cpu)
@@ -205,6 +219,8 @@ int main(int argc, char *argv[])
 	struct kvm_vcpu *vcpu;
 	u32 cpu, rseq_cpu;
 
+	init_vdso();
+
 	/* Tell stdout not to buffer its content */
 	setbuf(stdout, NULL);
 
@@ -253,7 +269,7 @@ int main(int argc, char *argv[])
 			 * across the seq_cnt reads.
 			 */
 			smp_rmb();
-			sys_getcpu(&cpu);
+			vdso_getcpu(&cpu, NULL, NULL);
 			rseq_cpu = rseq_current_cpu_raw();
 			smp_rmb();
 		} while (snapshot != atomic_read(&seq_cnt));
-- 
2.31.1