Save/restore UART state to/from snapshots#5764
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ilstam merged 4 commits intofirecracker-microvm:mainfrom Mar 31, 2026
Merged
Save/restore UART state to/from snapshots#5764ilstam merged 4 commits intofirecracker-microvm:mainfrom
ilstam merged 4 commits intofirecracker-microvm:mainfrom
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Type: Bug
Codecov Report❌ Patch coverage is Additional details and impacted files@@ Coverage Diff @@
## main #5764 +/- ##
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+ Coverage 83.02% 83.04% +0.02%
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Files 276 276
Lines 29430 29466 +36
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+ Hits 24435 24471 +36
Misses 4995 4995
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Manciukic
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let's add a changelog item as well for this as well
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just a couple of minor issues
The VMM does not save the UART state when taking a snapshot. Upon restoring it uses a weird hack in emulate_serial_init() where it enables RX (but no TX) interrupts after a snapshot restore. This hack apart form weird is also wrong as the guest driver might have never enabled RX interrupts in the first place. A comment in the same function acknowledges that the correct thing to do is save and restore the entire device state. Because we don't re-enable TX interrupts after a snapshot restore, the guest driver might hang if it relies on those TX interrupts as reported in [1]. Fix this by saving the UART state in snapshots and restoring it from them. Bump the SNAPSHOT_VERSION appropriately. [1] - firecracker-microvm#5730 Reported-by: EJ Ciramella <ejc3@meta.com> Signed-off-by: Ilias Stamatis <ilstam@amazon.com>
Add a simple unit test that injects a few bytes into the UART's FIFO, stores it's state, create a new device from the saved state and is able to read back the correct bytes. Signed-off-by: Ilias Stamatis <ilstam@amazon.com>
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Commit 7e619f4 ("devices: serial: Save/restore UART state to/from snapshots") fixed a bug where the guest driver would hang after a snapshot restore because Firecracker didn't properly restore the state of the UART device. Add a test_serial_active_tx_snapshot test case (which mostly duplicates the existing test_serial_after_snapshot). Start an unbounded transmission before taking the snapshot. This will saturate the TX buffer of the emulated UART and will make the guest driver enable TX interrupts. Without the fix, the test case fails because the driver is stuck waiting for a TX interrupt that will never be delivered after the snapshot is restored. Note that the serial.tx("") that sends a newline to flush the buffer in the original test case isn't needed anymore since sending Ctrl+C will achieve the same result. Signed-off-by: Ilias Stamatis <ilstam@amazon.com>
firecracker-microvm#5705 Signed-off-by: Ilias Stamatis <ilstam@amazon.com>
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The VMM does not save the UART state when taking a snapshot. Upon
restoring it uses a weird hack in emulate_serial_init() where it enables
RX (but no TX) interrupts after a snapshot restore. This hack apart form
weird is also wrong as the guest driver might have never enabled RX
interrupts in the first place. A comment in the same function
acknowledges that the correct thing to do is save and restore the entire
device state.
Because we don't re-enable TX interrupts after a snapshot restore, the
guest driver might hang if it relies on those TX interrupts as reported
in 1.
Fix this by saving the UART state in snapshots and restoring it from
them.
1 - #5730
License Acceptance
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CONTRIBUTING.md.PR Checklist
tools/devtool checkbuild --allto verify that the PR passesbuild checks on all supported architectures.
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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.