1/* SPDX-License-Identifier: ((GPL-2.0 WITH Linux-syscall-note) OR BSD-2-Clause) */
2
3/*
4 * This structure provides a vDSO-style clock to VM guests, exposing the
5 * relationship (or lack thereof) between the CPU clock (TSC, timebase, arch
6 * counter, etc.) and real time. It is designed to address the problem of
7 * live migration, which other clock enlightenments do not.
8 *
9 * When a guest is live migrated, this affects the clock in two ways.
10 *
11 * First, even between identical hosts the actual frequency of the underlying
12 * counter will change within the tolerances of its specification (typically
13 * ±50PPM, or 4 seconds a day). This frequency also varies over time on the
14 * same host, but can be tracked by NTP as it generally varies slowly. With
15 * live migration there is a step change in the frequency, with no warning.
16 *
17 * Second, there may be a step change in the value of the counter itself, as
18 * its accuracy is limited by the precision of the NTP synchronization on the
19 * source and destination hosts.
20 *
21 * So any calibration (NTP, PTP, etc.) which the guest has done on the source
22 * host before migration is invalid, and needs to be redone on the new host.
23 *
24 * In its most basic mode, this structure provides only an indication to the
25 * guest that live migration has occurred. This allows the guest to know that
26 * its clock is invalid and take remedial action. For applications that need
27 * reliable accurate timestamps (e.g. distributed databases), the structure
28 * can be mapped all the way to userspace. This allows the application to see
29 * directly for itself that the clock is disrupted and take appropriate
30 * action, even when using a vDSO-style method to get the time instead of a
31 * system call.
32 *
33 * In its more advanced mode. this structure can also be used to expose the
34 * precise relationship of the CPU counter to real time, as calibrated by the
35 * host. This means that userspace applications can have accurate time
36 * immediately after live migration, rather than having to pause operations
37 * and wait for NTP to recover. This mode does, of course, rely on the
38 * counter being reliable and consistent across CPUs.
39 *
40 * Note that this must be true UTC, never with smeared leap seconds. If a
41 * guest wishes to construct a smeared clock, it can do so. Presenting a
42 * smeared clock through this interface would be problematic because it
43 * actually messes with the apparent counter *period*. A linear smearing
44 * of 1 ms per second would effectively tweak the counter period by 1000PPM
45 * at the start/end of the smearing period, while a sinusoidal smear would
46 * basically be impossible to represent.
47 *
48 * This structure is offered with the intent that it be adopted into the
49 * nascent virtio-rtc standard, as a virtio-rtc that does not address the live
50 * migration problem seems a little less than fit for purpose. For that
51 * reason, certain fields use precisely the same numeric definitions as in
52 * the virtio-rtc proposal. The structure can also be exposed through an ACPI
53 * device with the CID "VMCLOCK", modelled on the "VMGENID" device except for
54 * the fact that it uses a real _CRS to convey the address of the structure
55 * (which should be a full page, to allow for mapping directly to userspace).
56 */
57
58#ifndef __VMCLOCK_ABI_H__
59#define __VMCLOCK_ABI_H__
60
61#include <linux/types.h>
62
63struct vmclock_abi {
64 /* CONSTANT FIELDS */
65 __le32 magic;
66#define VMCLOCK_MAGIC 0x4b4c4356 /* "VCLK" */
67 __le32 size; /* Size of region containing this structure */
68 __le16 version; /* 1 */
69 __u8 counter_id; /* Matches VIRTIO_RTC_COUNTER_xxx except INVALID */
70#define VMCLOCK_COUNTER_ARM_VCNT 0
71#define VMCLOCK_COUNTER_X86_TSC 1
72#define VMCLOCK_COUNTER_INVALID 0xff
73 __u8 time_type; /* Matches VIRTIO_RTC_TYPE_xxx */
74#define VMCLOCK_TIME_UTC 0 /* Since 1970-01-01 00:00:00z */
75#define VMCLOCK_TIME_TAI 1 /* Since 1970-01-01 00:00:00z */
76#define VMCLOCK_TIME_MONOTONIC 2 /* Since undefined epoch */
77#define VMCLOCK_TIME_INVALID_SMEARED 3 /* Not supported */
78#define VMCLOCK_TIME_INVALID_MAYBE_SMEARED 4 /* Not supported */
79
80 /* NON-CONSTANT FIELDS PROTECTED BY SEQCOUNT LOCK */
81 __le32 seq_count; /* Low bit means an update is in progress */
82 /*
83 * This field changes to another non-repeating value when the CPU
84 * counter is disrupted, for example on live migration. This lets
85 * the guest know that it should discard any calibration it has
86 * performed of the counter against external sources (NTP/PTP/etc.).
87 */
88 __le64 disruption_marker;
89 __le64 flags;
90 /* Indicates that the tai_offset_sec field is valid */
91#define VMCLOCK_FLAG_TAI_OFFSET_VALID (1 << 0)
92 /*
93 * Optionally used to notify guests of pending maintenance events.
94 * A guest which provides latency-sensitive services may wish to
95 * remove itself from service if an event is coming up. Two flags
96 * indicate the approximate imminence of the event.
97 */
98#define VMCLOCK_FLAG_DISRUPTION_SOON (1 << 1) /* About a day */
99#define VMCLOCK_FLAG_DISRUPTION_IMMINENT (1 << 2) /* About an hour */
100#define VMCLOCK_FLAG_PERIOD_ESTERROR_VALID (1 << 3)
101#define VMCLOCK_FLAG_PERIOD_MAXERROR_VALID (1 << 4)
102#define VMCLOCK_FLAG_TIME_ESTERROR_VALID (1 << 5)
103#define VMCLOCK_FLAG_TIME_MAXERROR_VALID (1 << 6)
104 /*
105 * If the MONOTONIC flag is set then (other than leap seconds) it is
106 * guaranteed that the time calculated according this structure at
107 * any given moment shall never appear to be later than the time
108 * calculated via the structure at any *later* moment.
109 *
110 * In particular, a timestamp based on a counter reading taken
111 * immediately after setting the low bit of seq_count (and the
112 * associated memory barrier), using the previously-valid time and
113 * period fields, shall never be later than a timestamp based on
114 * a counter reading taken immediately before *clearing* the low
115 * bit again after the update, using the about-to-be-valid fields.
116 */
117#define VMCLOCK_FLAG_TIME_MONOTONIC (1 << 7)
118
119 __u8 pad[2];
120 __u8 clock_status;
121#define VMCLOCK_STATUS_UNKNOWN 0
122#define VMCLOCK_STATUS_INITIALIZING 1
123#define VMCLOCK_STATUS_SYNCHRONIZED 2
124#define VMCLOCK_STATUS_FREERUNNING 3
125#define VMCLOCK_STATUS_UNRELIABLE 4
126
127 /*
128 * The time exposed through this device is never smeared. This field
129 * corresponds to the 'subtype' field in virtio-rtc, which indicates
130 * the smearing method. However in this case it provides a *hint* to
131 * the guest operating system, such that *if* the guest OS wants to
132 * provide its users with an alternative clock which does not follow
133 * UTC, it may do so in a fashion consistent with the other systems
134 * in the nearby environment.
135 */
136 __u8 leap_second_smearing_hint; /* Matches VIRTIO_RTC_SUBTYPE_xxx */
137#define VMCLOCK_SMEARING_STRICT 0
138#define VMCLOCK_SMEARING_NOON_LINEAR 1
139#define VMCLOCK_SMEARING_UTC_SLS 2
140 __le16 tai_offset_sec; /* Actually two's complement signed */
141 __u8 leap_indicator;
142 /*
143 * This field is based on the VIRTIO_RTC_LEAP_xxx values as defined
144 * in the current draft of virtio-rtc, but since smearing cannot be
145 * used with the shared memory device, some values are not used.
146 *
147 * The _POST_POS and _POST_NEG values allow the guest to perform
148 * its own smearing during the day or so after a leap second when
149 * such smearing may need to continue being applied for a leap
150 * second which is now theoretically "historical".
151 */
152#define VMCLOCK_LEAP_NONE 0x00 /* No known nearby leap second */
153#define VMCLOCK_LEAP_PRE_POS 0x01 /* Positive leap second at EOM */
154#define VMCLOCK_LEAP_PRE_NEG 0x02 /* Negative leap second at EOM */
155#define VMCLOCK_LEAP_POS 0x03 /* Set during 23:59:60 second */
156#define VMCLOCK_LEAP_POST_POS 0x04
157#define VMCLOCK_LEAP_POST_NEG 0x05
158
159 /* Bit shift for counter_period_frac_sec and its error rate */
160 __u8 counter_period_shift;
161 /*
162 * Paired values of counter and UTC at a given point in time.
163 */
164 __le64 counter_value;
165 /*
166 * Counter period, and error margin of same. The unit of these
167 * fields is 1/2^(64 + counter_period_shift) of a second.
168 */
169 __le64 counter_period_frac_sec;
170 __le64 counter_period_esterror_rate_frac_sec;
171 __le64 counter_period_maxerror_rate_frac_sec;
172
173 /*
174 * Time according to time_type field above.
175 */
176 __le64 time_sec; /* Seconds since time_type epoch */
177 __le64 time_frac_sec; /* Units of 1/2^64 of a second */
178 __le64 time_esterror_nanosec;
179 __le64 time_maxerror_nanosec;
180};
181
182#endif /* __VMCLOCK_ABI_H__ */
183