Public bug reported: Enabling hv-tlbflush on older hosts using Intel CPUs supporting VT-x but neither EPT nor VPID will lead to bluescreens on the guest.
It seems KVM only checks if EPT is available, and if it isn't it forcibly uses VPID. If that's *also* not available, it defaults to basically a no-op hypercall, though windows is expecting the TLB to be flushed. hv-tlbflush is pretty useless on machines not supporting these extensions anyway (only reasonably fix I can see would be to flush the *entire* TLB on tlbflush hypercall in KVM (i.e. a kernel fix), but that would remove any performance benefits), so I would suggest some kind of preliminary check and warning/error if hv-tlbflush is specified on such a host. All CPUs mentioned in this thread[0] are confirmed to be affected by the bug, and I have successfully reproduced it on an Intel Core2Duo E8500. [0] https://forum.proxmox.com/threads/windows-guest-bluescreen-with- proxmox-6.56053/ ** Affects: qemu Importance: Undecided Status: New ** Tags: qemu-kvm -- You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/1837851 Title: hv-tlbflush malfunctions on Intel host CPUs with neither EPT nor VPID (qemu-kvm) Status in QEMU: New Bug description: Enabling hv-tlbflush on older hosts using Intel CPUs supporting VT-x but neither EPT nor VPID will lead to bluescreens on the guest. It seems KVM only checks if EPT is available, and if it isn't it forcibly uses VPID. If that's *also* not available, it defaults to basically a no-op hypercall, though windows is expecting the TLB to be flushed. hv-tlbflush is pretty useless on machines not supporting these extensions anyway (only reasonably fix I can see would be to flush the *entire* TLB on tlbflush hypercall in KVM (i.e. a kernel fix), but that would remove any performance benefits), so I would suggest some kind of preliminary check and warning/error if hv-tlbflush is specified on such a host. All CPUs mentioned in this thread[0] are confirmed to be affected by the bug, and I have successfully reproduced it on an Intel Core2Duo E8500. [0] https://forum.proxmox.com/threads/windows-guest-bluescreen-with- proxmox-6.56053/ To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/1837851/+subscriptions
