Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 10 Jun 2020 17:15:34 -0700 | Subject | Re: [PATCH] Fix undefined operation VMXOFF during reboot and crash |
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On Wed, Jun 10, 2020 at 5:00 PM Sean Christopherson <sean.j.christopherson@intel.com> wrote: > > On Wed, Jun 10, 2020 at 02:59:19PM -0700, Andy Lutomirski wrote: > > > > > > > On Jun 10, 2020, at 11:21 AM, David P. Reed <dpreed@deepplum.com> wrote: > > > > > > If a panic/reboot occurs when CR4 has VMX enabled, a VMXOFF is > > > done on all CPUS, to allow the INIT IPI to function, since > > > INIT is suppressed when CPUs are in VMX root operation. > > > However, VMXOFF causes an undefined operation fault if the CPU is not > > > in VMX operation, that is, VMXON has not been executed, or VMXOFF > > > has been executed, but VMX is enabled. > > > > I’m surprised. Wouldn’t this mean that emergency reboots always fail it a VM > > is running? I would think someone would have noticed before. > > The call to cpu_vmxoff() is conditioned on CR4.VMXE==1, which KVM toggles in > tandem with VMXON and VMXOFF. Out of tree hypervisors presumably do the > same. That's obviously not atomic though, e.g. VMXOFF will #UD if the > vmxoff_nmi() NMI arrives between CR4.VMXE=1 and VMXON, or between VMXOFF > and CR4.VMXE=0.
It would be nice for the commit message to say "this happens when nmxoff_nmi() races with KVM's VMXON/VMXOFF toggling". Or the commit message should say something else if the bug happens for a different reason.
The race with KVM should be quite unusual, since it involves rebooting concurrently with loading or unloading KVM.
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