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Re: [REGRESSION][BISECTED] Long boot time with Xen HVM guests during PV spinlock initialization
- To: Teddy Astie <teddy.astie@xxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, regressions@xxxxxxxxxxxxxxx, Thomas Gleixner <tglx@xxxxxxxxxxxxx>
- From: Juergen Gross <jgross@xxxxxxxx>
- Date: Mon, 8 Jun 2026 12:57:46 +0200
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Olivier Lambert <olivier.lambert@xxxxxxxxxx>
- Delivery-date: Mon, 08 Jun 2026 10:57:49 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
Add Thomas Gleixner (author of the patch introducing the regression).
Juergen
On 08.06.26 12:29, Teddy Astie wrote:
Hello,
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. On this HVM guest, SMP bring-up runs while the
jiffies clocksource is still the timekeeper, and the Xen single shot (high
resolution) tick then advances jiffies far too slowly, so the secondary CPUs
burn seconds in calibrate_delay().
Teddy
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