fix(kvm-clock): do not jump monotonic clock on restore#5809
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Manciukic merged 3 commits intofirecracker-microvm:mainfrom Apr 7, 2026
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fix(kvm-clock): do not jump monotonic clock on restore#5809Manciukic merged 3 commits intofirecracker-microvm:mainfrom
Manciukic merged 3 commits intofirecracker-microvm:mainfrom
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Codecov Report❌ Patch coverage is
Additional details and impacted files@@ Coverage Diff @@
## main #5809 +/- ##
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- Coverage 83.00% 82.99% -0.01%
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Files 275 275
Lines 29383 29399 +16
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+ Hits 24388 24401 +13
- Misses 4995 4998 +3
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Removes trailing whitespace. Signed-off-by: Riccardo Mancini <mancio@amazon.com>
ilstam
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Firecracker has never advanced the clock on restore for any of the supported clocksources. Since Linux 5.16, the KVM_CLOCK_REALTIME has been passed to kvm-clock, causing the monotonic time in the guest to jump when using kvm-clock as clock source. Despite being unexpected and not what Firecracker should do, we recognize this may be a valid usecase so this patch adds a way to configure it, keeping the default to the expected documented behaviour. This patch adds a new API flag to LoadSnapshot, clock_realtime, that advances the clock on restore when set (default is False). Rather than the clock flags being decided at snapshot time, the restore path ignores those flags and decides what to do depending on the clock_realtime flag. This is because the other available flag (KVM_CLOCK_TSC_STABLE) cannot even be passed to `set_clock`, meaning the only valid flag is KVM_CLOCK_REALTIME. The name of the flag was kept generic as we may add this behaviour for the other clock sources in the future, if the need arises. Signed-off-by: Riccardo Mancini <mancio@amazon.com>
We actually support all of them. Signed-off-by: Riccardo Mancini <mancio@amazon.com>
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Changes
This patch adds a new API flag to LoadSnapshot, clock_realtime, that advances the clock on restore when set (default is False). Rather than the clock flags being decided at snapshot time, the restore path ignores those flags and decides what to do depending on the clock_realtime flag. This is because the other available flags are ignored (KVM_CLOCK_TSC_HOST) or cannot be passed to
set_clock(KVM_CLOCK_TSC_STABLE) , meaning the only valid flag is KVM_CLOCK_REALTIME.The name of the flag was kept generic as we may add this behaviour for the other clock sources in the future, if the need arises.
Reason
Firecracker has never advanced the clock on restore for any of the supported clocksources. Since Linux 5.16, the KVM_CLOCK_REALTIME has been passed to kvm-clock, causing the monotonic time in the guest to jump when using kvm-clock as clock source.
Despite being unexpected and not what Firecracker should do, we recognize this may be a valid usecase so this patch adds a way to configure it, keeping the default to the expected documented behaviour.
License Acceptance
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CONTRIBUTING.md.PR Checklist
tools/devtool checkbuild --allto verify that the PR passesbuild checks on all supported architectures.
tools/devtool checkstyleto verify that the PR passes theautomated style checks.
how they are solving the problem in a clear and encompassing way.
in the PR.
CHANGELOG.md.Runbook for Firecracker API changes.
integration tests.
TODO.rust-vmm.