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Step a cold-booted guest's clock to a host time stamped at its read - #1357
A cold-booted Firecracker VM came up 0.86 to 1.14 s behind the host. fc-agent stepped its clock to the host time written into MMDS while the boot plan was built, before the VM booted, so the guest landed on an instant about a second old, and chronyd took about 12 s to correct it.
Change
configure_and_boot_vm binds the boot-plan vsock listener on every backend. That listener stamps the host clock at each accept (serve_bootplan). Firecracker still publishes the plan via MMDS as well.
fc-agent sync_clock_from_host reads the vsock document on both transports and steps with set_system_clock, so Firecracker and Cloud Hypervisor share one boot clock path. The MMDS clock read is removed.
The step goes both ways. I first used Step a snapshot source's clock to the host's before its gate reopens #1355's advance_system_clock, and the new test failed: before the step, an arm64 guest's clock is the RTC's whole second plus the half second rtc_hctosys adds (tv_nsec = NSEC_PER_SEC >> 1), so it can be up to 0.5 s ahead of the host. One boot stayed 367 ms ahead. With a stamp taken at the guest's read, stepping in both directions is correct.
chronyd: the step still runs before fc-agent replaces the rootfs chronyd with its own, so the daemon measures the corrected clock and has no boot offset to carry.
snapshot run: a restored clone that rebooted now keeps the relaunch's listener handle, so the exit path aborts it. The handle was dropped before, and Firecracker now gets a listener on that relaunch too.
test_snapshot_of_a_relaunched_clone_leaves_it_running: after the relaunch the port serves the new guest's plan, so the check is now that the document carries no restore epoch. A stale restore epoch is what that test guards against.
Contract: a cold-booted guest's clock is within 100 ms of the host's at healthy, on both backends, with no chronyd correction pending from the boot step. This touches the production boot path. A failed vsock read still falls back to chronyd, as before.
test_cold_boot_clock_matches_the_host (new, test_clone_restore_fixes.rs) boots a rootless --no-snapshot VM and samples it 20 times right after healthy, each guest read bracketed between two host reads, with a 100 ms tolerance.
tree
worst offset
main
862.5 ms, 849 ms (both tries)
fix
0.0 ms outside the bracket
fix with the fc-agent half reverted
873.0 ms, 924.5 ms
fix, 5 repeat runs (boot steps -0.22 to +0.49 s)
0.0 ms each
the_boot_step_moves_the_clock_back_too (fc-agent source pin) fails when sync_clock_from_host uses advance_system_clock.
make lint clean
make test-unit FILTER="-p fc-agent" 291 passed
make test-unit FILTER="-E 'test(bootplan) | test(listener) | test(relaunch)'" 62 passed
make test-root FILTER="-E 'binary(test_clone_restore_fixes) | binary(test_snapshot_clone)
| test(test_snapshot_of_a_relaunched_clone_leaves_it_running)
| (binary(test_sanity) & test(vsock))'" 64 passed
cargo check -p fc-agent --target {aarch64,x86_64}-unknown-linux-musl clean
Summary by CodeRabbit
Bug Fixes
Improved guest clock accuracy during boot across supported backends. Guests now use a fresh host timestamp, helping reduce clock drift on cold starts and after relaunches. The clock can be adjusted either forward or backward to better match the host time.
A Firecracker VM that cold-booted came up 0.86 to 1.14 s behind the host
(issue 1354). fc-agent stepped its clock to the host time written into
MMDS while the boot plan was built, before the VM booted, so the guest
landed on an instant about a second old and chronyd took about 12 s to
correct it. Cloud Hypervisor already read the plan over vsock, where the
host stamps its clock at each accept.
- vm_config.rs: configure_and_boot_vm binds the boot-plan vsock listener
on every backend; Firecracker still publishes the plan via MMDS too.
- fc-agent bootplan.rs: sync_clock_from_host reads the vsock document on
both transports and steps with set_system_clock. The step goes both
ways: before it an arm64 guest's clock is the RTC's whole second plus
the half second rtc_hctosys adds, up to 0.5 s ahead of the host, and a
forward-only step (advance_system_clock) left one boot 367 ms ahead.
The step still runs before fc-agent starts its chronyd, so the daemon
measures the corrected clock and has no boot offset to carry.
- fc-agent mmds.rs: the MMDS clock read is removed (no caller left).
- snapshot.rs: a restored clone that rebooted keeps the relaunch's
listener handle, so the exit path aborts it; it was dropped before.
- test_reboot.rs: after that relaunch the port serves the new guest's
plan, so the check is that it carries no restore epoch, which is the
hazard the test guards.
- test_clone_restore_fixes.rs: the source-clock test no longer waits
for chronyd to fix the cold boot before snapshotting.
Tests:
- test_cold_boot_clock_matches_the_host (new, integration): a rootless
--no-snapshot cold boot, sampled 20 times right after healthy with
host-bracketed reads, tolerance 100 ms. Without the fix: 862.5 ms
and 849 ms behind on both tries; again 873.0 and 924.5 ms with the
fc-agent half reverted. With it: 0.0 ms outside the bracket.
- the_boot_step_moves_the_clock_back_too (fc-agent, source pin): fails
with advance_system_clock in sync_clock_from_host.
The boot-plan vsock listener now runs for both backends and serves host-time metadata. fc-agent reads the timestamp and elapsed read duration to set the guest clock. Tests cover cold-boot clock offset and the boot-plan document served after relaunch.
Changes: Boot-time clock synchronization
Layer / File(s)
Summary
Listener setup and lifecycle src/commands/podman/types.rs, src/commands/podman/vm_config.rs, src/commands/podman/listeners.rs, src/commands/snapshot.rs
The boot-plan listener starts for both backends. Firecracker still publishes the plan through MMDS when configured to do so. The listener handle is retained through VM setup and successful relaunch.
fc-agent now reads host-time metadata from vsock on both transports, measures elapsed time after the read, and calls set_system_clock. The MMDS clock-sync function is removed.
Clock and reboot validation tests/test_clone_restore_fixes.rs, tests/test_reboot.rs
A cold-boot test checks that the maximum sampled guest-clock offset is at most 100 ms. Reboot validation now fails if the relaunched VM’s boot-plan document contains restore-epoch.
sequenceDiagram
participant Host as Host boot setup
participant Listener as Boot-plan vsock listener
participant Agent as fc-agent
participant Clock as Guest system clock
Host->>Listener: Start listener with host-time metadata
Agent->>Listener: Read boot-plan document
Listener-->>Agent: Return host-time metadata
Agent->>Clock: set_system_clock(host timestamp, elapsed duration)
Loading
Possibly related issues
Issue 1344 describes cold-boot clock lag from reading a timestamp set during configuration. This change serves host-time metadata through the boot-plan vsock listener for the guest to read.
Merge Risk: Low · up to 5466c
A delayed clock read can miss the 100 ms target, and the relaunch test would not catch a missing listener. These bounded risks should be addressed or explicitly accepted before merging.
Pre-merge checks: 5
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Explanation
Description Check
Check skipped - CodeRabbit’s high-level summary is enabled.
Title check
The title clearly and concisely describes the primary change: stepping a cold-booted guest clock to a host timestamp captured at read time.
Linked Issues check
Directly linked issue #1354 requires that a cold-booted guest stamp host time at its read, not at boot-plan build, and that a correct clock not be stepped back to a stale stamp. At head, `configure_an…
Out of Scope Changes check
The changes stay within the clock-sync objective of #1354 and its supporting work. Each change connects to the listener-on-every-backend design or to tests that check it. `src/commands/podman/vm_confi…
Docstring Coverage
Docstring coverage is 88.89% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Documentation-only edits and deletions within surviv…
Full details: Linked Issues check
Explanation
Directly linked issue #1354 requires that a cold-booted guest stamp host time at its read, not at boot-plan build, and that a correct clock not be stepped back to a stale stamp. At head, configure_and_boot_vm binds the boot-plan vsock listener on every backend (per the PR summary and vm_config.rs change). The listener stamps host time at each accept, so the stamp is taken at the guest read. This matches the issue's suggested vsock time query. In fc-agent/src/bootplan.rs, sync_clock_from_host reads the vsock document on both transports, times the read, and calls set_system_clock(&meta.host_time_ns, read.elapsed()). The MMDS clock read is removed from fc-agent/src/mmds.rs. The backward step is allowed because the stamp is fresh. The code comment cites the arm64 RTC half-second offset as the reason for a two-way step. The source test the_boot_step_moves_the_clock_back_too checks this in code. Restore and Cloud Hypervisor behavior is not changed in a way the issue excludes. The 100 ms cold-boot contract is supported by test_cold_boot_clock_matches_the_host, which is present in the diff. The before-and-after measurements (862.5 ms, 849 ms on main; 0.0 ms with fix) are author-reported. They were not rerun during this assessment. Fallback to chronyd on vsock failure is kept, as the PR states.
Full details: Out of Scope Changes check
Explanation
The changes stay within the clock-sync objective of #1354 and its supporting work. Each change connects to the listener-on-every-backend design or to tests that check it. src/commands/podman/vm_config.rs starts the listener for all backends and publishes the plan through MMDS only when bootplan_over_vsock is false. src/commands/snapshot.rs keeps the relaunch listener handle so the exit path aborts it, because the listener now runs on that relaunch too. tests/test_reboot.rs changes its expectation: the port now serves a document without restore-epoch after relaunch, which follows from that listener change. tests/test_clone_restore_fixes.rs adds the cold-boot clock test and drops the wait for chronyd in the snapshot-source test. That change follows from the earlier boot step. Documentation updates in fc-agent/src/mmds.rs, listeners.rs, types.rs, and firecracker/config.rs describe the new behavior. The removed MMDS sync_clock_from_host has no remaining caller in the summary, and agent.rs is updated to match. No unrelated functional change was found in the summarized diff.
Full details: Docstring Coverage
Explanation
Docstring coverage is 88.89% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 9 functions across 9 files.
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served_boot_plan returns None when the listener is absent. The current if let Some skips the epoch check in that case. The other relaunch assertions check machine-id, health, container state, snapshots, and VM execution, but not the boot-plan listener.
Suggested fix
- if let Some(document) = served_boot_plan(clone_pid)- .await?- .filter(|document| document.get("restore-epoch").is_some())- {+ let document = served_boot_plan(clone_pid)+ .await?+ .context("the relaunched clone's boot-plan port served no document")?;+ if document.get("restore-epoch").is_some() {
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Review comment at @tests/test_reboot.rs around lines 788 - 790:
Update the relaunch assertion using served_boot_plan so a missing boot-plan
document fails the test instead of skipping the restore-epoch check; add a clear
context message when the result is None, then check restore-epoch on the
required document.
🪄 Fix comments: Fix CodeRabbit comments on this PR
Prompt to fix review comments
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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.
Inline comments:
Review comments at @fc-agent/src/bootplan.rs:
- Around line 115-116: Update the timing around read_vsock_doc so elapsed time
for the clock adjustment includes the document read; capture the start timestamp
as close to vsock connection establishment as possible and use it when adjusting
the system clock.
---
Nitpick comments:
Review comments at @tests/test_reboot.rs:
- Around line 788-790: Update the relaunch assertion using served_boot_plan so a
missing boot-plan document fails the test instead of skipping the restore-epoch
check; add a clear context message when the result is None, then check
restore-epoch on the required document.
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Commits
Reviewing files that changed from the base of the PR and between aa1af46 and 5466c5b.
Files selected for processing: 10
fc-agent/src/agent.rs
fc-agent/src/bootplan.rs
fc-agent/src/mmds.rs
src/commands/podman/listeners.rs
src/commands/podman/types.rs
src/commands/podman/vm_config.rs
src/commands/snapshot.rs
src/firecracker/config.rs
tests/test_clone_restore_fixes.rs
tests/test_reboot.rs
Files with no reviewable changes: 1
fc-agent/src/mmds.rs
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DISAGREE: not blocking; tracked in #1358. The inline finding (count the document read in the clock step) is answered in its thread: the stamp falls between the guest's connect and the end of its read, so either end of that window bounds the error by the read's duration, and a cold boot measured 0.0 ms outside the host bracket in 7 runs. The tests/test_reboot.rs nitpick (require a document after the relaunch) hardens a test whose job is to catch a stale restore epoch, which it still does. Both are listed in #1358. The walkthrough summarizes the diff and raises nothing beyond these two.
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Fixes #1354.
A cold-booted Firecracker VM came up 0.86 to 1.14 s behind the host. fc-agent stepped its clock to the host time written into MMDS while the boot plan was built, before the VM booted, so the guest landed on an instant about a second old, and chronyd took about 12 s to correct it.
Change
configure_and_boot_vmbinds the boot-plan vsock listener on every backend. That listener stamps the host clock at each accept (serve_bootplan). Firecracker still publishes the plan via MMDS as well.sync_clock_from_hostreads the vsock document on both transports and steps withset_system_clock, so Firecracker and Cloud Hypervisor share one boot clock path. The MMDS clock read is removed.advance_system_clock, and the new test failed: before the step, an arm64 guest's clock is the RTC's whole second plus the half secondrtc_hctosysadds (tv_nsec = NSEC_PER_SEC >> 1), so it can be up to 0.5 s ahead of the host. One boot stayed 367 ms ahead. With a stamp taken at the guest's read, stepping in both directions is correct.snapshot run: a restored clone that rebooted now keeps the relaunch's listener handle, so the exit path aborts it. The handle was dropped before, and Firecracker now gets a listener on that relaunch too.test_snapshot_of_a_relaunched_clone_leaves_it_running: after the relaunch the port serves the new guest's plan, so the check is now that the document carries no restore epoch. A stale restore epoch is what that test guards against.test_source_clock_matches_the_host_after_snapshot_create(Step a snapshot source's clock to the host's before its gate reopens #1355) no longer waits for chronyd to correct the cold boot before snapshotting.Contract and evidence
Contract: a cold-booted guest's clock is within 100 ms of the host's at healthy, on both backends, with no chronyd correction pending from the boot step. This touches the production boot path. A failed vsock read still falls back to chronyd, as before.
test_cold_boot_clock_matches_the_host(new,test_clone_restore_fixes.rs) boots a rootless--no-snapshotVM and samples it 20 times right after healthy, each guest read bracketed between two host reads, with a 100 ms tolerance.the_boot_step_moves_the_clock_back_too(fc-agent source pin) fails whensync_clock_from_hostusesadvance_system_clock.Summary by CodeRabbit