Re: [PATCH] Fix undefined operation VMXOFF during reboot and crash

[Sean Christopherson]

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@xxxxxxxxxxxx> wrote:
> > 
> > ïIf 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.
> 
> Iâm surprised. Wouldnât 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.