KVM: selftests: Get VM's binary stats FD when opening VM

Get and cache a VM's binary stats FD when the VM is opened, as opposed to
waiting until the stats are first used.  Opening the stats FD outside of
__vm_get_stat() will allow converting it to a scope-agnostic helper.

Note, this doesn't interfere with kvm_binary_stats_test's testcase that
verifies a stats FD can be used after its own VM's FD is closed, as the
cached FD is also closed during kvm_vm_free().

Link: https://lore.kernel.org/r/20250111005049.1247555-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kvm/lib/kvm_util.c

@@ -196,6 +196,11 @@ static void vm_open(struct kvm_vm *vm)
 
 	vm->fd = __kvm_ioctl(vm->kvm_fd, KVM_CREATE_VM, (void *)vm->type);
 	TEST_ASSERT(vm->fd >= 0, KVM_IOCTL_ERROR(KVM_CREATE_VM, vm->fd));
+
+	if (kvm_has_cap(KVM_CAP_BINARY_STATS_FD))
+		vm->stats.fd = vm_get_stats_fd(vm);
+	else
+		vm->stats.fd = -1;
 }
 
 const char *vm_guest_mode_string(uint32_t i)
@@ -661,14 +666,17 @@ static void kvm_stats_release(struct kvm_binary_stats *stats)
 {
 	int ret;
 
-	if (!stats->desc)
+	if (stats->fd < 0)
 		return;
 
-	free(stats->desc);
-	stats->desc = NULL;
+	if (stats->desc) {
+		free(stats->desc);
+		stats->desc = NULL;
+	}
 
 	ret = close(stats->fd);
 	TEST_ASSERT(!ret,  __KVM_SYSCALL_ERROR("close()", ret));
+	stats->fd = -1;
 }
 
 __weak void vcpu_arch_free(struct kvm_vcpu *vcpu)
@@ -2231,7 +2239,6 @@ void __vm_get_stat(struct kvm_vm *vm, const char *name, uint64_t *data,
 	int i;
 
 	if (!stats->desc) {
-		stats->fd = vm_get_stats_fd(vm);
 		read_stats_header(stats->fd, &stats->header);
 		stats->desc = read_stats_descriptors(stats->fd, &stats->header);
 	}