fix(snapshot/x86_64): make sure TSC_DEADLINE MSR is non-zero #4618
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Codecov ReportAll modified and coverable lines are covered by tests ✅
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## main #4618 +/- ##
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On x86_64, we observed that when restoring from a snapshot, one of the vCPUs had MSR_IA32_TSC_DEADLINE cleared and never received TSC interrupts until the MSR is updated externally (eg by setting the system time). We believe this happens because the TSC interrupt is lost during snapshot taking process: the MSR is cleared, but the interrupt is not delivered to the guest, so the guest does not rearm the timer. A visible effect of that is failure to connect to a restored VM via SSH. This commit introduces a workaround. If when taking a snapshot, we see a zero MSR_IA32_TSC_DEADLINE, we replace its value with the MSR_IA32_TSC value from the same vCPU to make sure that the vCPU will continue to receive TSC interrupts. Signed-off-by: Nikita Kalyazin <[email protected]>
The TSC_DEADLINE MSR value is volatile is it is getting updated by the guest kernel based on the current TSC value. Signed-off-by: Nikita Kalyazin <[email protected]>
The TSC_DEADLINE MSR value is volatile is it is getting updated by the guest kernel based on the current TSC value. Signed-off-by: Nikita Kalyazin <[email protected]>
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Changes
This change introduces a workaround. If when taking a snapshot, we see a zero MSR_IA32_TSC_DEADLINE, we replace its value with the MSR_IA32_TSC value from the same vCPU to make sure the vCPU will continue to receive TSC interrupts.
Reason
On x86_64, we observed that when restoring from a snapshot, one of the vCPUs had MSR_IA32_TSC_DEADLINE cleared and never received TSC interrupts until the MSR is updated externally (eg by setting the system time).
We believe this happens because the TSC interrupt is lost during snapshot taking process: the MSR is cleared, but the interrupt is not delivered to the guest, so the guest does not rearm the timer.
A visible effect of that is failure to connect to a restored VM via SSH, similar to https://buildkite.com/firecracker/firecracker-pr-nightly/builds/1403#018f83db-5395-4656-8d9c-83b6fcfcfd54/50-1994 .
License Acceptance
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Certificate of Origin and signing off your commits, please check
CONTRIBUTING.md.PR Checklist
[ ] If a specific issue led to this PR, this PR closes the issue.PR.
[ ] API changes follow the Runbook for Firecracker API changes.CHANGELOG.md.[ ] NewTODOs link to an issue.contribution quality standards.
rust-vmm.