1// SPDX-License-Identifier: GPL-2.0-only
2/*
3 * KVM binary statistics interface implementation
4 *
5 * Copyright 2021 Google LLC
6 */
7
8#include <linux/kvm_host.h>
9#include <linux/kvm.h>
10#include <linux/errno.h>
11#include <linux/uaccess.h>
12
13/**
14 * kvm_stats_read() - Common function to read from the binary statistics
15 * file descriptor.
16 *
17 * @id: identification string of the stats
18 * @header: stats header for a vm or a vcpu
19 * @desc: start address of an array of stats descriptors for a vm or a vcpu
20 * @stats: start address of stats data block for a vm or a vcpu
21 * @size_stats: the size of stats data block pointed by @stats
22 * @user_buffer: start address of userspace buffer
23 * @size: requested read size from userspace
24 * @offset: the start position from which the content will be read for the
25 *          corresponding vm or vcp file descriptor
26 *
27 * The file content of a vm/vcpu file descriptor is now defined as below:
28 * +-------------+
29 * |   Header    |
30 * +-------------+
31 * |  id string  |
32 * +-------------+
33 * | Descriptors |
34 * +-------------+
35 * | Stats Data  |
36 * +-------------+
37 * Although this function allows userspace to read any amount of data (as long
38 * as in the limit) from any position, the typical usage would follow below
39 * steps:
40 * 1. Read header from offset 0. Get the offset of descriptors and stats data
41 *    and some other necessary information. This is a one-time work for the
42 *    lifecycle of the corresponding vm/vcpu stats fd.
43 * 2. Read id string from its offset. This is a one-time work for the lifecycle
44 *    of the corresponding vm/vcpu stats fd.
45 * 3. Read descriptors from its offset and discover all the stats by parsing
46 *    descriptors. This is a one-time work for the lifecycle of the
47 *    corresponding vm/vcpu stats fd.
48 * 4. Periodically read stats data from its offset using pread.
49 *
50 * Return: the number of bytes that has been successfully read
51 */
52ssize_t kvm_stats_read(char *id, const struct kvm_stats_header *header,
53		       const struct _kvm_stats_desc *desc,
54		       void *stats, size_t size_stats,
55		       char __user *user_buffer, size_t size, loff_t *offset)
56{
57	ssize_t len;
58	ssize_t copylen;
59	ssize_t remain = size;
60	size_t size_desc;
61	size_t size_header;
62	void *src;
63	loff_t pos = *offset;
64	char __user *dest = user_buffer;
65
66	size_header = sizeof(*header);
67	size_desc = header->num_desc * sizeof(*desc);
68
69	len = KVM_STATS_NAME_SIZE + size_header + size_desc + size_stats - pos;
70	len = min(len, remain);
71	if (len <= 0)
72		return 0;
73	remain = len;
74
75	/*
76	 * Copy kvm stats header.
77	 * The header is the first block of content userspace usually read out.
78	 * The pos is 0 and the copylen and remain would be the size of header.
79	 * The copy of the header would be skipped if offset is larger than the
80	 * size of header. That usually happens when userspace reads stats
81	 * descriptors and stats data.
82	 */
83	copylen = size_header - pos;
84	copylen = min(copylen, remain);
85	if (copylen > 0) {
86		src = (void *)header + pos;
87		if (copy_to_user(dest, src, copylen))
88			return -EFAULT;
89		remain -= copylen;
90		pos += copylen;
91		dest += copylen;
92	}
93
94	/*
95	 * Copy kvm stats header id string.
96	 * The id string is unique for every vm/vcpu, which is stored in kvm
97	 * and kvm_vcpu structure.
98	 * The id string is part of the stat header from the perspective of
99	 * userspace, it is usually read out together with previous constant
100	 * header part and could be skipped for later descriptors and stats
101	 * data readings.
102	 */
103	copylen = header->id_offset + KVM_STATS_NAME_SIZE - pos;
104	copylen = min(copylen, remain);
105	if (copylen > 0) {
106		src = id + pos - header->id_offset;
107		if (copy_to_user(dest, src, copylen))
108			return -EFAULT;
109		remain -= copylen;
110		pos += copylen;
111		dest += copylen;
112	}
113
114	/*
115	 * Copy kvm stats descriptors.
116	 * The descriptors copy would be skipped in the typical case that
117	 * userspace periodically read stats data, since the pos would be
118	 * greater than the end address of descriptors
119	 * (header->header.desc_offset + size_desc) causing copylen <= 0.
120	 */
121	copylen = header->desc_offset + size_desc - pos;
122	copylen = min(copylen, remain);
123	if (copylen > 0) {
124		src = (void *)desc + pos - header->desc_offset;
125		if (copy_to_user(dest, src, copylen))
126			return -EFAULT;
127		remain -= copylen;
128		pos += copylen;
129		dest += copylen;
130	}
131
132	/* Copy kvm stats values */
133	copylen = header->data_offset + size_stats - pos;
134	copylen = min(copylen, remain);
135	if (copylen > 0) {
136		src = stats + pos - header->data_offset;
137		if (copy_to_user(dest, src, copylen))
138			return -EFAULT;
139		pos += copylen;
140	}
141
142	*offset = pos;
143	return len;
144}
145