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Diffstat (limited to 'include/uapi/linux/vmclock-abi.h')
| -rw-r--r-- | include/uapi/linux/vmclock-abi.h | 182 |
1 files changed, 182 insertions, 0 deletions
diff --git a/include/uapi/linux/vmclock-abi.h b/include/uapi/linux/vmclock-abi.h new file mode 100644 index 000000000000..2d99b29ac44a --- /dev/null +++ b/include/uapi/linux/vmclock-abi.h @@ -0,0 +1,182 @@ +/* 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) + + __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; +}; + +#endif /* __VMCLOCK_ABI_H__ */ |