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+/* SPDX-License-Identifier: ((GPL-2.0 WITH Linux-syscall-note) OR BSD-2-Clause) */
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+
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+/*
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+ * This structure provides a vDSO-style clock to VM guests, exposing the
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+ * relationship (or lack thereof) between the CPU clock (TSC, timebase, arch
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+ * counter, etc.) and real time. It is designed to address the problem of
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+ * live migration, which other clock enlightenments do not.
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+ *
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+ * When a guest is live migrated, this affects the clock in two ways.
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+ *
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+ * First, even between identical hosts the actual frequency of the underlying
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+ * counter will change within the tolerances of its specification (typically
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+ * ±50PPM, or 4 seconds a day). This frequency also varies over time on the
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+ * same host, but can be tracked by NTP as it generally varies slowly. With
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+ * live migration there is a step change in the frequency, with no warning.
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+ *
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+ * Second, there may be a step change in the value of the counter itself, as
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+ * its accuracy is limited by the precision of the NTP synchronization on the
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+ * source and destination hosts.
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+ *
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+ * So any calibration (NTP, PTP, etc.) which the guest has done on the source
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+ * host before migration is invalid, and needs to be redone on the new host.
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+ *
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+ * In its most basic mode, this structure provides only an indication to the
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+ * guest that live migration has occurred. This allows the guest to know that
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+ * its clock is invalid and take remedial action. For applications that need
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+ * reliable accurate timestamps (e.g. distributed databases), the structure
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+ * can be mapped all the way to userspace. This allows the application to see
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+ * directly for itself that the clock is disrupted and take appropriate
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+ * action, even when using a vDSO-style method to get the time instead of a
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+ * system call.
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+ *
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+ * In its more advanced mode. this structure can also be used to expose the
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+ * precise relationship of the CPU counter to real time, as calibrated by the
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+ * host. This means that userspace applications can have accurate time
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+ * immediately after live migration, rather than having to pause operations
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+ * and wait for NTP to recover. This mode does, of course, rely on the
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+ * counter being reliable and consistent across CPUs.
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+ *
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+ * Note that this must be true UTC, never with smeared leap seconds. If a
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+ * guest wishes to construct a smeared clock, it can do so. Presenting a
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+ * smeared clock through this interface would be problematic because it
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+ * actually messes with the apparent counter *period*. A linear smearing
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+ * of 1 ms per second would effectively tweak the counter period by 1000PPM
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+ * at the start/end of the smearing period, while a sinusoidal smear would
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+ * basically be impossible to represent.
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+ *
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+ * This structure is offered with the intent that it be adopted into the
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+ * nascent virtio-rtc standard, as a virtio-rtc that does not address the live
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+ * migration problem seems a little less than fit for purpose. For that
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+ * reason, certain fields use precisely the same numeric definitions as in
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+ * the virtio-rtc proposal. The structure can also be exposed through an ACPI
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+ * device with the CID "VMCLOCK", modelled on the "VMGENID" device except for
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+ * the fact that it uses a real _CRS to convey the address of the structure
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+ * (which should be a full page, to allow for mapping directly to userspace).
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+ */
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+
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+#ifndef __VMCLOCK_ABI_H__
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+#define __VMCLOCK_ABI_H__
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+
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+#include "standard-headers/linux/types.h"
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+
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+struct vmclock_abi {
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+ /* CONSTANT FIELDS */
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+ uint32_t magic;
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+#define VMCLOCK_MAGIC 0x4b4c4356 /* "VCLK" */
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+ uint32_t size; /* Size of region containing this structure */
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+ uint16_t version; /* 1 */
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+ uint8_t counter_id; /* Matches VIRTIO_RTC_COUNTER_xxx except INVALID */
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+#define VMCLOCK_COUNTER_ARM_VCNT 0
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+#define VMCLOCK_COUNTER_X86_TSC 1
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+#define VMCLOCK_COUNTER_INVALID 0xff
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+ uint8_t time_type; /* Matches VIRTIO_RTC_TYPE_xxx */
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+#define VMCLOCK_TIME_UTC 0 /* Since 1970-01-01 00:00:00z */
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+#define VMCLOCK_TIME_TAI 1 /* Since 1970-01-01 00:00:00z */
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+#define VMCLOCK_TIME_MONOTONIC 2 /* Since undefined epoch */
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+#define VMCLOCK_TIME_INVALID_SMEARED 3 /* Not supported */
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+#define VMCLOCK_TIME_INVALID_MAYBE_SMEARED 4 /* Not supported */
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+
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+ /* NON-CONSTANT FIELDS PROTECTED BY SEQCOUNT LOCK */
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+ uint32_t seq_count; /* Low bit means an update is in progress */
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+ /*
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+ * This field changes to another non-repeating value when the CPU
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+ * counter is disrupted, for example on live migration. This lets
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+ * the guest know that it should discard any calibration it has
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+ * performed of the counter against external sources (NTP/PTP/etc.).
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+ */
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+ uint64_t disruption_marker;
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+ uint64_t flags;
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+ /* Indicates that the tai_offset_sec field is valid */
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+#define VMCLOCK_FLAG_TAI_OFFSET_VALID (1 << 0)
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+ /*
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+ * Optionally used to notify guests of pending maintenance events.
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+ * A guest which provides latency-sensitive services may wish to
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+ * remove itself from service if an event is coming up. Two flags
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+ * indicate the approximate imminence of the event.
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+ */
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+#define VMCLOCK_FLAG_DISRUPTION_SOON (1 << 1) /* About a day */
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+#define VMCLOCK_FLAG_DISRUPTION_IMMINENT (1 << 2) /* About an hour */
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+#define VMCLOCK_FLAG_PERIOD_ESTERROR_VALID (1 << 3)
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+#define VMCLOCK_FLAG_PERIOD_MAXERROR_VALID (1 << 4)
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+#define VMCLOCK_FLAG_TIME_ESTERROR_VALID (1 << 5)
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+#define VMCLOCK_FLAG_TIME_MAXERROR_VALID (1 << 6)
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+ /*
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+ * If the MONOTONIC flag is set then (other than leap seconds) it is
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+ * guaranteed that the time calculated according this structure at
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+ * any given moment shall never appear to be later than the time
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+ * calculated via the structure at any *later* moment.
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+ *
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+ * In particular, a timestamp based on a counter reading taken
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+ * immediately after setting the low bit of seq_count (and the
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+ * associated memory barrier), using the previously-valid time and
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+ * period fields, shall never be later than a timestamp based on
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+ * a counter reading taken immediately before *clearing* the low
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+ * bit again after the update, using the about-to-be-valid fields.
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+ */
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+#define VMCLOCK_FLAG_TIME_MONOTONIC (1 << 7)
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+
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+ uint8_t pad[2];
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+ uint8_t clock_status;
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+#define VMCLOCK_STATUS_UNKNOWN 0
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+#define VMCLOCK_STATUS_INITIALIZING 1
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+#define VMCLOCK_STATUS_SYNCHRONIZED 2
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+#define VMCLOCK_STATUS_FREERUNNING 3
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+#define VMCLOCK_STATUS_UNRELIABLE 4
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+
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+ /*
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+ * The time exposed through this device is never smeared. This field
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+ * corresponds to the 'subtype' field in virtio-rtc, which indicates
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+ * the smearing method. However in this case it provides a *hint* to
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+ * the guest operating system, such that *if* the guest OS wants to
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+ * provide its users with an alternative clock which does not follow
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+ * UTC, it may do so in a fashion consistent with the other systems
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+ * in the nearby environment.
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+ */
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+ uint8_t leap_second_smearing_hint; /* Matches VIRTIO_RTC_SUBTYPE_xxx */
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+#define VMCLOCK_SMEARING_STRICT 0
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+#define VMCLOCK_SMEARING_NOON_LINEAR 1
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+#define VMCLOCK_SMEARING_UTC_SLS 2
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+ uint16_t tai_offset_sec; /* Actually two's complement signed */
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+ uint8_t leap_indicator;
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+ /*
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+ * This field is based on the VIRTIO_RTC_LEAP_xxx values as defined
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+ * in the current draft of virtio-rtc, but since smearing cannot be
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+ * used with the shared memory device, some values are not used.
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+ *
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+ * The _POST_POS and _POST_NEG values allow the guest to perform
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+ * its own smearing during the day or so after a leap second when
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+ * such smearing may need to continue being applied for a leap
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+ * second which is now theoretically "historical".
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+ */
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+#define VMCLOCK_LEAP_NONE 0x00 /* No known nearby leap second */
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+#define VMCLOCK_LEAP_PRE_POS 0x01 /* Positive leap second at EOM */
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+#define VMCLOCK_LEAP_PRE_NEG 0x02 /* Negative leap second at EOM */
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+#define VMCLOCK_LEAP_POS 0x03 /* Set during 23:59:60 second */
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+#define VMCLOCK_LEAP_POST_POS 0x04
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+#define VMCLOCK_LEAP_POST_NEG 0x05
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+
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+ /* Bit shift for counter_period_frac_sec and its error rate */
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+ uint8_t counter_period_shift;
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+ /*
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+ * Paired values of counter and UTC at a given point in time.
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+ */
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+ uint64_t counter_value;
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+ /*
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+ * Counter period, and error margin of same. The unit of these
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+ * fields is 1/2^(64 + counter_period_shift) of a second.
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+ */
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+ uint64_t counter_period_frac_sec;
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+ uint64_t counter_period_esterror_rate_frac_sec;
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+ uint64_t counter_period_maxerror_rate_frac_sec;
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+
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+ /*
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+ * Time according to time_type field above.
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+ */
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+ uint64_t time_sec; /* Seconds since time_type epoch */
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+ uint64_t time_frac_sec; /* Units of 1/2^64 of a second */
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+ uint64_t time_esterror_nanosec;
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+ uint64_t time_maxerror_nanosec;
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+};
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+
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+#endif /* __VMCLOCK_ABI_H__ */
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David Woodhouse