Booting on GCE: "WARNING: Processor claims to not support invariant TSC"

[anttikantee]

• https://www.freelists.org/post/rumpkernel-users/First-boot-on-Google-Compute-Engine
• https://gist.github.com/englishm/52db261c84fc941df322#file-gistfile1-txt-L327

Either the test is broken or timekeeping on GCE is broken (or both).

[mato]

Cross-pasting from mailing list for the record:

The test is not broken. KVM will not export an invariant TSC unless explicitly told to do so, and it also implies disabling migration.

Try it yourself -- on a TSC invariant machine the relevant qemu runes are "-cpu host,migratable=no,+invtsc". Without those you will keep getting the warning.

As to whether or not timekeeping is actually broken in the case where the test fails, hard to say. Tentative answer is "probably, but need to actually do some long-running scientific testing to find out".

The way KVM pvclock is designed it seems to imply the guest should use TSC regardless, but presumably with some kind of $BACKUP_PARANOIA. More investigation needed.