Re: [RFC PATCH v2] ptp: Add vDSO-style vmclock support

Peter Hilber
2024-06-28T11:33:35Z

On 27.06.24 18:03, David Woodhouse wrote:
>
> 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 {

[...]

>
> /*
> * 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 */

Nitpick: The comment is not valid any more for TIME_MONOTONIC.