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A guest clock drifting away from the host clock, and a hypercall pulling it back

A VM's Clock Cannot Be Trusted β€” and the ptp_kvm Fix for QEMU/KVM

A virtual machine’s clock is built on an assumption virtualisation breaks: that the CPU keeps counting. Why guest time drifts on every hypervisor, what the skew actually breaks β€” Kerberos, TLS, Ceph, Windows β€” and how ptp_kvm fixes it properly on QEMU/KVM.

7th August 2026 Β· 14 min Β· 2897 words Β· Damien Dye