linux/virt/kvm/binary_stats.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * KVM binary statistics interface implementation
 *
 * Copyright 2021 Google LLC
 */

#include <linux/kvm_host.h>
#include <linux/kvm.h>
#include <linux/errno.h>
#include <linux/uaccess.h>

/**
 * kvm_stats_read() - Common function to read from the binary statistics
 * file descriptor.
 *
 * @id: identification string of the stats
 * @header: stats header for a vm or a vcpu
 * @desc: start address of an array of stats descriptors for a vm or a vcpu
 * @stats: start address of stats data block for a vm or a vcpu
 * @size_stats: the size of stats data block pointed by @stats
 * @user_buffer: start address of userspace buffer
 * @size: requested read size from userspace
 * @offset: the start position from which the content will be read for the
 *          corresponding vm or vcp file descriptor
 *
 * The file content of a vm/vcpu file descriptor is now defined as below:
 * +-------------+
 * |   Header    |
 * +-------------+
 * |  id string  |
 * +-------------+
 * | Descriptors |
 * +-------------+
 * | Stats Data  |
 * +-------------+
 * Although this function allows userspace to read any amount of data (as long
 * as in the limit) from any position, the typical usage would follow below
 * steps:
 * 1. Read header from offset 0. Get the offset of descriptors and stats data
 *    and some other necessary information. This is a one-time work for the
 *    lifecycle of the corresponding vm/vcpu stats fd.
 * 2. Read id string from its offset. This is a one-time work for the lifecycle
 *    of the corresponding vm/vcpu stats fd.
 * 3. Read descriptors from its offset and discover all the stats by parsing
 *    descriptors. This is a one-time work for the lifecycle of the
 *    corresponding vm/vcpu stats fd.
 * 4. Periodically read stats data from its offset using pread.
 *
 * Return: the number of bytes that has been successfully read
 */
ssize_t kvm_stats_read(char *id, const struct kvm_stats_header *header,
		       const struct _kvm_stats_desc *desc,
		       void *stats, size_t size_stats,
		       char __user *user_buffer, size_t size, loff_t *offset)
{ … }