I've updated the tree at https://git.infradead.org/users/dwmw2/linux.git/shortlog/refs/heads/vmclock (but not yet the qemu one). I think I've taken into account all your comments apart from the one about non-64-bit counters wrapping. I reduced the seq_count to 32 bit to make room for a 32-bit flags field, added the time type (UTC/TAI/MONOTONIC) and a smearing hint, with some straw man definitions for smearing algorithms for which I could actually find definitions. The structure now looks like this: struct vmclock_abi { uint32_t magic; #define VMCLOCK_MAGIC 0x4b4c4356 /* "VCLK" */ uint16_t size; /* Size of page containing this structure */ uint16_t version; /* 1 */ /* Sequence lock. Low bit means an update is in progress. */ uint32_t seq_count; uint32_t 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 may wish to remove itself from service if an event is * coming up. Two flags indicate the rough imminence of the event. */ #define VMCLOCK_FLAG_DISRUPTION_SOON (1 << 1) /* About a day */ #define VMCLOCK_FLAG_DISRUPTION_IMMINENT (1 << 2) /* About an hour */ /* Indicates that the utc_time_maxerror_picosec field is valid */ #define VMCLOCK_FLAG_UTC_MAXERROR_VALID (1 << 3) /* Indicates counter_period_error_rate_frac_sec is valid */ #define VMCLOCK_FLAG_PERIOD_ERROR_VALID (1 << 4) /* * 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.). */ uint64_t disruption_marker; uint8_t 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 uint8_t counter_id; #define VMCLOCK_COUNTER_INVALID 0 #define VMCLOCK_COUNTER_X86_TSC 1 #define VMCLOCK_COUNTER_ARM_VCNT 2 #define VMCLOCK_COUNTER_X86_ART 3 /* * By providing the offset from UTC to TAI, the guest can know both * UTC and TAI reliably, whichever is indicated in the time_type * field. Valid if VMCLOCK_FLAG_TAI_OFFSET_VALID is set in flags. */ int16_t tai_offset_sec; /* * The time exposed through this device is never smeaared; if it * claims to be VMCLOCK_TIME_UTC then it MUST be UTC. This field * 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 the POSIX CLOCK_REALTIME standard, * it may do so in a fashion consistent with the other systems * in the nearby environment. */ uint8_t leap_second_smearing_hint; /* Provide true UTC to users, unsmeared. */; #define VMCLOCK_SMEARING_NONE 0 /* * https://aws.amazon.com/blogs/aws/look-before-you-leap-the-coming-leap-second-and-aws/ * From noon on the day before to noon on the day after, smear the * clock by a linear 1/86400s per second. */ #define VMCLOCK_SMEARING_LINEAR_86400 1 /* * draft-kuhn-leapsecond-00 * For the 1000s leading up to the leap second, smear the clock by * clock by a linear 1ms per second. */ #define VMCLOCK_SMEARING_UTC_SLS 2 /* * What time is exposed in the time_sec/time_frac_sec fields? */ uint8_t time_type; #define VMCLOCK_TIME_UNKNOWN 0 /* Invalid / no time exposed */ #define VMCLOCK_TIME_UTC 1 /* Since 1970-01-01 00:00:00z */ #define VMCLOCK_TIME_TAI 2 /* Since 1970-01-01 00:00:00z */ #define VMCLOCK_TIME_MONOTONIC 3 /* Since undefined epoch */ /* Bit shift for counter_period_frac_sec and its error rate */ uint8_t counter_period_shift; /* * Unlike in NTP, this can indicate a leap second in the past. This * is needed to allow guests to derive an imprecise clock with * smeared leap seconds for themselves, as some modes of smearing * need the adjustments to continue even after the moment at which * the leap second should have occurred. */ int8_t leapsecond_direction; uint64_t leapsecond_tai_sec; /* Since 1970-01-01 00:00:00z */ /* * Paired values of counter and UTC at a given point in time. */ uint64_t counter_value; uint64_t time_sec; /* Since 1970-01-01 00:00:00z */ uint64_t time_frac_sec; /* * Counter frequency, and error margin. The unit of these fields is * seconds >> (64 + counter_period_shift) */ uint64_t counter_period_frac_sec; uint64_t counter_period_error_rate_frac_sec; /* Error margin of UTC reading above (± picoseconds) */ uint64_t utc_time_maxerror_picosec; }; #endif /* __VMCLOCK_H__ */
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