Re: [PATCH v3 03/18] x86/reboot: KVM: Handle VMXOFF in KVM's reboot callback

[Sean Christopherson <seanjc@google.com>]

On Mon, May 22, 2023, Kai Huang wrote:
> On Fri, 2023-05-12 at 16:50 -0700, Sean Christopherson wrote:
> > Use KVM VMX's reboot/crash callback to do VMXOFF in an emergency instead
> > of manually and blindly doing VMXOFF.  There's no need to attempt VMXOFF
> > if a hypervisor, i.e. KVM, isn't loaded/active, i.e. if the CPU can't
> > possibly be post-VMXON.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index fc9cdb4114cc..76cdb189f1b5 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -744,7 +744,7 @@ static int vmx_set_guest_uret_msr(struct vcpu_vmx *vmx,
> >  	return ret;
> >  }
> >  
> > -static void crash_vmclear_local_loaded_vmcss(void)
> > +static void vmx_emergency_disable(void)
> >  {
> >  	int cpu = raw_smp_processor_id();
> >  	struct loaded_vmcs *v;
> > @@ -752,6 +752,8 @@ static void crash_vmclear_local_loaded_vmcss(void)
> >  	list_for_each_entry(v, &per_cpu(loaded_vmcss_on_cpu, cpu),
> >  			    loaded_vmcss_on_cpu_link)
> >  		vmcs_clear(v->vmcs);
> > +
> > +	__cpu_emergency_vmxoff();
> 
> __cpu_emergency_vmxoff() internally checks whether VMX is enabled in CR4.  
> Logically, looks it's more reasonable to do such check before VMCLEAR active
> VMCSes, although in practice there should be no problem I think.
> 
> But this problem inherits from the existing code in  upstream, so not sure
> whether it is worth fixing.

Hmm, I think it's worth fixing, if only to avoid confusing future readers.  Blindly
doing VMCLEAR but then conditionally executing VMXOFF is nonsensical.  I'll tack on
a patch, and also add a comment to call out that CR4.VMXE can be _cleared_
asynchronously by NMI, but can't be set after being checked.  I.e. explain that
checking CR4.VMXE is a "best effort" sort of thing.