/* SPDX-License-Identifier: ((GPL-2.0 WITH Linux-syscall-note) OR BSD-2-Clause) */ /* * This structure provides a vDSO-style clock to VM guests, exposing the * relationship (or lack thereof) between the CPU clock (TSC, timebase, arch * counter, etc.) and real time. It is designed to address the problem of * live migration, which other clock enlightenments do not. * * When a guest is live migrated, this affects the clock in two ways. * * First, even between identical hosts the actual frequency of the underlying * counter will change within the tolerances of its specification (typically * ±50PPM, or 4 seconds a day). This frequency also varies over time on the * same host, but can be tracked by NTP as it generally varies slowly. With * live migration there is a step change in the frequency, with no warning. * * Second, there may be a step change in the value of the counter itself, as * its accuracy is limited by the precision of the NTP synchronization on the * source and destination hosts. * * So any calibration (NTP, PTP, etc.) which the guest has done on the source * host before migration is invalid, and needs to be redone on the new host. * * In its most basic mode, this structure provides only an indication to the * guest that live migration has occurred. This allows the guest to know that * its clock is invalid and take remedial action. For applications that need * reliable accurate timestamps (e.g. distributed databases), the structure * can be mapped all the way to userspace. This allows the application to see * directly for itself that the clock is disrupted and take appropriate * action, even when using a vDSO-style method to get the time instead of a * system call. * * In its more advanced mode. this structure can also be used to expose the * precise relationship of the CPU counter to real time, as calibrated by the * host. This means that userspace applications can have accurate time * immediately after live migration, rather than having to pause operations * and wait for NTP to recover. This mode does, of course, rely on the * counter being reliable and consistent across CPUs. * * Note that this must be true UTC, never with smeared leap seconds. If a * guest wishes to construct a smeared clock, it can do so. Presenting a * smeared clock through this interface would be problematic because it * actually messes with the apparent counter *period*. A linear smearing * of 1 ms per second would effectively tweak the counter period by 1000PPM * at the start/end of the smearing period, while a sinusoidal smear would * basically be impossible to represent. * * This structure is offered with the intent that it be adopted into the * nascent virtio-rtc standard, as a virtio-rtc that does not address the live * migration problem seems a little less than fit for purpose. For that * reason, certain fields use precisely the same numeric definitions as in * the virtio-rtc proposal. The structure can also be exposed through an ACPI * device with the CID "VMCLOCK", modelled on the "VMGENID" device except for * the fact that it uses a real _CRS to convey the address of the structure * (which should be a full page, to allow for mapping directly to userspace). */ #ifndef __VMCLOCK_ABI_H__ #define __VMCLOCK_ABI_H__ #include <linux/types.h> struct vmclock_abi { /* CONSTANT FIELDS */ __le32 magic; #define VMCLOCK_MAGIC 0x4b4c4356 /* "VCLK" */ __le32 size; /* Size of region containing this structure */ __le16 version; /* 1 */ __u8 counter_id; /* Matches VIRTIO_RTC_COUNTER_xxx except INVALID */ #define VMCLOCK_COUNTER_ARM_VCNT 0 #define VMCLOCK_COUNTER_X86_TSC 1 #define VMCLOCK_COUNTER_INVALID 0xff __u8 time_type; /* Matches VIRTIO_RTC_TYPE_xxx */ #define VMCLOCK_TIME_UTC 0 /* Since 1970-01-01 00:00:00z */ #define VMCLOCK_TIME_TAI 1 /* Since 1970-01-01 00:00:00z */ #define VMCLOCK_TIME_MONOTONIC 2 /* Since undefined epoch */ #define VMCLOCK_TIME_INVALID_SMEARED 3 /* Not supported */ #define VMCLOCK_TIME_INVALID_MAYBE_SMEARED 4 /* Not supported */ /* NON-CONSTANT FIELDS PROTECTED BY SEQCOUNT LOCK */ __le32 seq_count; /* Low bit means an update is in progress */ /* * This field changes to another non-repeating value when the CPU * counter is disrupted, for example on live migration. This lets * the guest know that it should discard any calibration it has * performed of the counter against external sources (NTP/PTP/etc.). */ __le64 disruption_marker; __le64 flags; /* Indicates that the tai_offset_sec field is valid */ #define VMCLOCK_FLAG_TAI_OFFSET_VALID (1 << 0) /* * Optionally used to notify guests of pending maintenance events. * A guest which provides latency-sensitive services may wish to * remove itself from service if an event is coming up. Two flags * indicate the approximate imminence of the event. */ #define VMCLOCK_FLAG_DISRUPTION_SOON (1 << 1) /* About a day */ #define VMCLOCK_FLAG_DISRUPTION_IMMINENT (1 << 2) /* About an hour */ #define VMCLOCK_FLAG_PERIOD_ESTERROR_VALID (1 << 3) #define VMCLOCK_FLAG_PERIOD_MAXERROR_VALID (1 << 4) #define VMCLOCK_FLAG_TIME_ESTERROR_VALID (1 << 5) #define VMCLOCK_FLAG_TIME_MAXERROR_VALID (1 << 6) /* * If the MONOTONIC flag is set then (other than leap seconds) it is * guaranteed that the time calculated according this structure at * any given moment shall never appear to be later than the time * calculated via the structure at any *later* moment. * * In particular, a timestamp based on a counter reading taken * immediately after setting the low bit of seq_count (and the * associated memory barrier), using the previously-valid time and * period fields, shall never be later than a timestamp based on * a counter reading taken immediately before *clearing* the low * bit again after the update, using the about-to-be-valid fields. */ #define VMCLOCK_FLAG_TIME_MONOTONIC (1 << 7) /* * If the VM_GEN_COUNTER_PRESENT flag is set, the hypervisor will * bump the vm_generation_counter field every time the guest is * loaded from some save state (restored from a snapshot). */ #define VMCLOCK_FLAG_VM_GEN_COUNTER_PRESENT (1 << 8) /* * If the NOTIFICATION_PRESENT flag is set, the hypervisor will send * a notification every time it updates seq_count to a new even number. */ #define VMCLOCK_FLAG_NOTIFICATION_PRESENT (1 << 9) __u8 pad[2]; __u8 clock_status; #define VMCLOCK_STATUS_UNKNOWN 0 #define VMCLOCK_STATUS_INITIALIZING 1 #define VMCLOCK_STATUS_SYNCHRONIZED 2 #define VMCLOCK_STATUS_FREERUNNING 3 #define VMCLOCK_STATUS_UNRELIABLE 4 /* * The time exposed through this device is never smeared. This field * corresponds to the 'subtype' field in virtio-rtc, which indicates * the smearing method. However in this case it provides a *hint* to * the guest operating system, such that *if* the guest OS wants to * provide its users with an alternative clock which does not follow * UTC, it may do so in a fashion consistent with the other systems * in the nearby environment. */ __u8 leap_second_smearing_hint; /* Matches VIRTIO_RTC_SUBTYPE_xxx */ #define VMCLOCK_SMEARING_STRICT 0 #define VMCLOCK_SMEARING_NOON_LINEAR 1 #define VMCLOCK_SMEARING_UTC_SLS 2 __le16 tai_offset_sec; /* Actually two's complement signed */ __u8 leap_indicator; /* * This field is based on the VIRTIO_RTC_LEAP_xxx values as defined * in the current draft of virtio-rtc, but since smearing cannot be * used with the shared memory device, some values are not used. * * The _POST_POS and _POST_NEG values allow the guest to perform * its own smearing during the day or so after a leap second when * such smearing may need to continue being applied for a leap * second which is now theoretically "historical". */ #define VMCLOCK_LEAP_NONE 0x00 /* No known nearby leap second */ #define VMCLOCK_LEAP_PRE_POS 0x01 /* Positive leap second at EOM */ #define VMCLOCK_LEAP_PRE_NEG 0x02 /* Negative leap second at EOM */ #define VMCLOCK_LEAP_POS 0x03 /* Set during 23:59:60 second */ #define VMCLOCK_LEAP_POST_POS 0x04 #define VMCLOCK_LEAP_POST_NEG 0x05 /* Bit shift for counter_period_frac_sec and its error rate */ __u8 counter_period_shift; /* * Paired values of counter and UTC at a given point in time. */ __le64 counter_value; /* * Counter period, and error margin of same. The unit of these * fields is 1/2^(64 + counter_period_shift) of a second. */ __le64 counter_period_frac_sec; __le64 counter_period_esterror_rate_frac_sec; __le64 counter_period_maxerror_rate_frac_sec; /* * Time according to time_type field above. */ __le64 time_sec; /* Seconds since time_type epoch */ __le64 time_frac_sec; /* Units of 1/2^64 of a second */ __le64 time_esterror_nanosec; __le64 time_maxerror_nanosec; /* * This field changes to another non-repeating value when the guest * has been loaded from a snapshot. In addition to handling a * disruption in time (which will also be signalled through the * disruption_marker field), a guest may wish to discard UUIDs, * reset network connections, reseed entropy, etc. */ __le64 vm_generation_counter; }; #endif /* __VMCLOCK_ABI_H__ */
