Re: [REGRESSION][BISECTED] Long boot time with Xen HVM guests during PV spinlock initialization

[Thomas Gleixner <tglx@xxxxxxxxxx>]

On Mon, Jun 08 2026 at 17:13, Thomas Gleixner wrote:
> On Mon, Jun 08 2026 at 12:29, Teddy Astie wrote:
>> In 6.12.5+ kernels on AMD CPUs, we observe abnormally long boot times 
>> where the guest is struggling on PV spinlock initialization.
>>
>> This occurs starting with 6.12.5, and also on more recent kernels on 
>> Intel platforms, but that hasn't been fully investigated at this time 
>> (but I assume it's a variant of the same issue).
>>
>> This occurs since a backport of 76031d9 ("clocksource: Make negative 
>> motion detection more robust").
>>
>> Some (claude-based) analysis made appears to relate that to the lack of 
>> proper max_raw_delta in the jiffies clocksource which appears to make 
>> the clock fail to progress meaningfully.
>>
>> Here is a raw summary of the analysis
>>  > We tracked it down to a single stable backport in 6.12.5: commit 
>> 1a678f6829a8 ("clocksource: Make negative motion detection more robust", 
>> upstream 76031d9536a0). It introduces a max_raw_delta field on struct 
>> clocksource but never initializes it for the default boot timekeeper 
>> (the jiffies clocksource), so clocksource_delta() clamps every delta to 
>> 0 and CLOCK_MONOTONIC freezes while that clocksource is active.
>
> Bah. jiffies clocksource is registered way _after_ timekeeping started to
> use it.
>
> The untested below should fix that.

That obviously needs to be:

--- a/kernel/time/jiffies.c
+++ b/kernel/time/jiffies.c
@@ -60,15 +60,9 @@ EXPORT_SYMBOL(get_jiffies_64);
 
 EXPORT_SYMBOL(jiffies);
 
-static int __init init_jiffies_clocksource(void)
-{
-       return __clocksource_register(&clocksource_jiffies);
-}
-
-core_initcall(init_jiffies_clocksource);
-
 struct clocksource * __init __weak clocksource_default_clock(void)
 {
+       __clocksource_register(&clocksource_jiffies);
        return &clocksource_jiffies;
 }