vmm: Synchronize vCPU TSC on restore#8437
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Jun 19, 2026
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- hypervisor: kvm: Reduce fully qualified paths
- vmm: cpu: Re-sync vCPU TSC offsets after restore
Reduce, but don't fully eliminate the fully qualified std:: paths. Signed-off-by: Rob Bradford <rbradford@meta.com>
Restoring a snapshot (snapshot restore or live-migration receive) sets each vCPU's TSC by writing MSR_IA32_TSC as the vCPU is created. However because CpuManager creates and restores vCPUs one at a time, the host TSC advances between the per-vCPU writes and KVM derives a slightly different TSC offset for each vCPU. KVM only engages its masterclock when every offset matches. This has a side effect of breaking the HyperV TSC reference clock page resulting in significantly reduced performance on Windows. After restore synchronise all vCPU's TSC offset to the boot vCPU's via the KVM_VCPU_TSC_CTRL device attribute group (Linux 5.16+) this allows the KVM TSC masterclock to engage and mitigates performance issues with the KVM HyperV emulation. See: cloud-hypervisor#8383 Signed-off-by: Rob Bradford <rbradford@meta.com> Assisted-by: Claude <claude-opus-4-8>
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@tonicmuroq I think this should also mitigate the issue that you were trying to fix in #8383 |
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| #[cfg(target_arch = "x86_64")] | ||
| ioctl_io_nr!(KVM_NMI, kvm_bindings::KVMIO, 0x9a); | ||
| // kvm-ioctls only exposes the vCPU device-attribute ioctls for aarch64. |
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Can't we fix this in kvm-ioctls?
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