KVM: selftests: Read binary stat data in lib

Move the code to read the binary stats data to the KVM selftests
library. It will be re-used by other tests to check KVM behavior.

Also opportunistically remove an unnecessary calculation with
"size_data" in stats_test.

Reviewed-by: David Matlack <dmatlack@google.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20220613212523.3436117-6-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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

@@ -341,6 +341,10 @@ static inline struct kvm_stats_desc *get_stats_descriptor(struct kvm_stats_desc
 	return (void *)stats + index * get_stats_descriptor_size(header);
 }
 
+void read_stat_data(int stats_fd, struct kvm_stats_header *header,
+		    struct kvm_stats_desc *desc, uint64_t *data,
+		    size_t max_elements);
+
 void vm_create_irqchip(struct kvm_vm *vm);
 
 void vm_set_user_memory_region(struct kvm_vm *vm, uint32_t slot, uint32_t flags,

tools/testing/selftests/kvm/kvm_binary_stats_test.c

@@ -147,15 +147,10 @@ static void stats_test(int stats_fd)
 	ret = pread(stats_fd, stats_data, size_data, header.data_offset);
 	TEST_ASSERT(ret == size_data, "Read KVM stats data");
 	/* Read kvm stats data one by one */
-	size_data = 0;
 	for (i = 0; i < header.num_desc; ++i) {
 		pdesc = get_stats_descriptor(stats_desc, i, &header);
-		ret = pread(stats_fd, stats_data,
-				pdesc->size * sizeof(*stats_data),
-				header.data_offset + size_data);
-		TEST_ASSERT(ret == pdesc->size * sizeof(*stats_data),
-				"Read data of KVM stats: %s", pdesc->name);
-		size_data += pdesc->size * sizeof(*stats_data);
+		read_stat_data(stats_fd, &header, pdesc, stats_data,
+			       pdesc->size);
 	}
 
 	free(stats_data);

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

@@ -1883,3 +1883,38 @@ struct kvm_stats_desc *read_stats_descriptors(int stats_fd,
 
 	return stats_desc;
 }
+
+/*
+ * Read stat data for a particular stat
+ *
+ * Input Args:
+ *   stats_fd - the file descriptor for the binary stats file from which to read
+ *   header - the binary stats metadata header corresponding to the given FD
+ *   desc - the binary stat metadata for the particular stat to be read
+ *   max_elements - the maximum number of 8-byte values to read into data
+ *
+ * Output Args:
+ *   data - the buffer into which stat data should be read
+ *
+ * Read the data values of a specified stat from the binary stats interface.
+ */
+void read_stat_data(int stats_fd, struct kvm_stats_header *header,
+		    struct kvm_stats_desc *desc, uint64_t *data,
+		    size_t max_elements)
+{
+	size_t nr_elements = min_t(ssize_t, desc->size, max_elements);
+	size_t size = nr_elements * sizeof(*data);
+	ssize_t ret;
+
+	TEST_ASSERT(desc->size, "No elements in stat '%s'", desc->name);
+	TEST_ASSERT(max_elements, "Zero elements requested for stat '%s'", desc->name);
+
+	ret = pread(stats_fd, data, size,
+		    header->data_offset + desc->offset);
+
+	TEST_ASSERT(ret >= 0, "pread() failed on stat '%s', errno: %i (%s)",
+		    desc->name, errno, strerror(errno));
+	TEST_ASSERT(ret == size,
+		    "pread() on stat '%s' read %ld bytes, wanted %lu bytes",
+		    desc->name, size, ret);
+}