KVM: selftests: Add struct and helpers to wrap binary stats cache

Add a struct and helpers to manage the binary stats cache, which is
currently used only for VM-scoped stats.  This will allow expanding the
selftests infrastructure to provide support for vCPU-scoped binary stats,
which, except for the ioctl to get the stats FD are identical to VM-scoped
stats.

Defer converting __vm_get_stat() to a scope-agnostic helper to a future
patch, as getting the stats FD from KVM needs to be moved elsewhere
before it can be made completely scope-agnostic.

Link: https://lore.kernel.org/r/20250111005049.1247555-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kvm/include/kvm_util.h

@@ -46,6 +46,12 @@ struct userspace_mem_region {
 	struct hlist_node slot_node;
 };
 
+struct kvm_binary_stats {
+	int fd;
+	struct kvm_stats_header header;
+	struct kvm_stats_desc *desc;
+};
+
 struct kvm_vcpu {
 	struct list_head list;
 	uint32_t id;
@@ -99,10 +105,7 @@ struct kvm_vm {
 
 	struct kvm_vm_arch arch;
 
-	/* Cache of information for binary stats interface */
-	int stats_fd;
-	struct kvm_stats_header stats_header;
-	struct kvm_stats_desc *stats_desc;
+	struct kvm_binary_stats stats;
 
 	/*
 	 * KVM region slots. These are the default memslots used by page

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

@@ -657,6 +657,20 @@ userspace_mem_region_find(struct kvm_vm *vm, uint64_t start, uint64_t end)
 	return NULL;
 }
 
+static void kvm_stats_release(struct kvm_binary_stats *stats)
+{
+	int ret;
+
+	if (!stats->desc)
+		return;
+
+	free(stats->desc);
+	stats->desc = NULL;
+
+	ret = close(stats->fd);
+	TEST_ASSERT(!ret,  __KVM_SYSCALL_ERROR("close()", ret));
+}
+
 __weak void vcpu_arch_free(struct kvm_vcpu *vcpu)
 {
 
@@ -711,13 +725,7 @@ void kvm_vm_release(struct kvm_vm *vmp)
 	TEST_ASSERT(!ret,  __KVM_SYSCALL_ERROR("close()", ret));
 
 	/* Free cached stats metadata and close FD */
-	if (vmp->stats_desc) {
-		free(vmp->stats_desc);
-		vmp->stats_desc = NULL;
-
-		ret = close(vmp->stats_fd);
-		TEST_ASSERT(!ret,  __KVM_SYSCALL_ERROR("close()", ret));
-	}
+	kvm_stats_release(&vmp->stats);
 }
 
 static void __vm_mem_region_delete(struct kvm_vm *vm,
@@ -2214,34 +2222,33 @@ void read_stat_data(int stats_fd, struct kvm_stats_header *header,
  *
  * Read the data values of a specified stat from the binary stats interface.
  */
-void __vm_get_stat(struct kvm_vm *vm, const char *stat_name, uint64_t *data,
+void __vm_get_stat(struct kvm_vm *vm, const char *name, uint64_t *data,
 		   size_t max_elements)
 {
+	struct kvm_binary_stats *stats = &vm->stats;
 	struct kvm_stats_desc *desc;
 	size_t size_desc;
 	int i;
 
-	if (!vm->stats_desc) {
-		vm->stats_fd = vm_get_stats_fd(vm);
-		read_stats_header(vm->stats_fd, &vm->stats_header);
-		vm->stats_desc = read_stats_descriptors(vm->stats_fd,
-							&vm->stats_header);
+	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);
 	}
 
-	size_desc = get_stats_descriptor_size(&vm->stats_header);
+	size_desc = get_stats_descriptor_size(&stats->header);
 
-	for (i = 0; i < vm->stats_header.num_desc; ++i) {
-		desc = (void *)vm->stats_desc + (i * size_desc);
+	for (i = 0; i < stats->header.num_desc; ++i) {
+		desc = (void *)stats->desc + (i * size_desc);
 
-		if (strcmp(desc->name, stat_name))
+		if (strcmp(desc->name, name))
 			continue;
 
-		read_stat_data(vm->stats_fd, &vm->stats_header, desc,
-			       data, max_elements);
+		read_stat_data(stats->fd, &stats->header, desc, data, max_elements);
 		return;
 	}
 
-	TEST_FAIL("Unable to find stat '%s'", stat_name);
+	TEST_FAIL("Unable to find stat '%s'", name);
 }
 
 __weak void kvm_arch_vm_post_create(struct kvm_vm *vm)