Re: [PATCH 1/5] x86/time: deal with negative deltas in get_s_time_fixed()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.01.2026 09:50, Jan Beulich wrote:
> On 15.01.2026 09:00, Jan Beulich wrote:
>> On 14.01.2026 18:49, Roger Pau Monné wrote:
>>> On Tue, Jan 06, 2026 at 02:58:11PM +0100, Jan Beulich wrote:
>>>> stime2tsc() guards against negative deltas by using 0 instead; I'm not
>>>> quite sure that's correct either.
>>>
>>> Hm, we should likely do the same for stime2tsc() that you do for
>>> get_s_time_fixed().  Given the current callers I think we might be
>>> safe, but it's a risk.
>>
>> Will do then.
> 
> While doing so, I came to wonder if there isn't a reason for this "capping".
> In local_time_calibration() we also have
> 
>     /* Local time warps forward if it lags behind master time. */
>     if ( curr.local_stime < curr.master_stime )
>         curr.local_stime = curr.master_stime;
> 
> Which for the use of stime2tsc() in cstate_restore_tsc() might mean that
> indeed there is a worry of the delta being negative, and the desire to
> "warp forward" in that case.

Proposed new function implementation (easier to look at than the diff):

uint64_t stime2tsc(s_time_t stime)
{
    const struct cpu_time *t = &this_cpu(cpu_time);
    s_time_t stime_delta = stime - t->stamp.local_stime;
    int64_t delta = 0;

    /*
     * While for reprogram_timer() the capping at 0 isn't relevant (the returned
     * value is likely in the past anyway then, by the time it is used), for
     * cstate_restore_tsc() it is meaningful: We need to avoid moving the TSC
     * backwards (relative to when it may last have been read).
     */
    if ( stime_delta > 0 )
    {
        struct time_scale sys_to_tsc = scale_reciprocal(t->tsc_scale);

        delta = scale_delta(stime_delta, &sys_to_tsc);
    }

    return t->stamp.local_tsc + delta;
}

Jan