Linux kernel (arch/x86/kvm/vmx): KVM's guest/host-mode Intel PT virtualization was broken end to end and the

Impact

KVM's guest/host-mode Intel PT virtualization was broken end to end and the maintainers say the bugs put host stability and health at risk. KVM trusts the guest's CPUID configuration to decide which RTIT MSRs to save and load, so a VM enumerating more address ranges than hardware supports drives the host into passing through, saving and loading non-existent MSRs - WARN storms, ToPA errors on the host, and a potential host deadlock. The feature was disabled outright rather than fixed.

Who can reach it

Requires the host to run kvm_intel with pt_mode=host_guest, which is not the default, and the VMM to expose Intel PT to the guest. Given that, the guest's own CPUID and RTIT_CTL programming drives the broken host paths from inside the VM. Nodes left on the default system-wide PT mode are not exposed.

What to do

Update to a kernel where PT guest/host mode is buried behind CONFIG_BROKEN. Interim and permanent control: never set kvm_intel.pt_mode=host_guest, and keep Intel PT out of tenant guest CPUID.

References