Re: [PATCH 2/3] KVM: nVMX: restore L1's EFER prior to setting the nested state

[Maxim Levitsky]

On Wed, 2021-11-10 at 16:01 +0100, Paolo Bonzini wrote:
> On 11/10/21 11:00, Maxim Levitsky wrote:
> > +	/*
> > +	 * The vcpu might currently contain L2's IA32_EFER, due to the way
> > +	 * some userspace kvm users (e.g qemu) restore nested state.
> > +	 *
> > +	 * To fix this, restore its IA32_EFER to the value it would have
> > +	 * after VM exit from the nested guest.
> > +	 *
> > +	 */
> > +
> > +	vcpu->arch.efer = nested_vmx_get_vmcs12_host_efer(vcpu, vmcs12);
> > +
> 
> In principle the value of LOAD_HOST_EFER on exit need not be the same as 
> on entry.  But you don't need all of EFER, only EFER.LME/EFER.LMA, and 
> those two bits must match ("the values of the LMA and LME bits in the 
> field must each be that of the “host address-space size” VM-exit 
> control" from the "Checks on Host Control Registers, MSRs, and SSP"; 
> plus the "Checks Related to Address-Space Size").
> 
> At least it's worth adjusting the comment to explain that.  But the root 
> cause of the issue is just nested_vmx_check_* accessing vcpu->arch.  So 
> you can instead:
> 
> - split out of nested_vmx_check_host_state a new function 
> nested_vmx_check_address_state_size that does
> 
> #ifdef CONFIG_X86_64
> 	if (CC(!!(vmcs12->vm_exit_controls & VM_EXIT_HOST_ADDR_SPACE_SIZE) !=
> 	       !!(vcpu->arch.efer & EFER_LMA)))
> 		return -EINVAL;
> #endif
> 	return 0;
> 
> - call it from vmentry but not from migration
> 
> - in nested_vmx_check_host_state, assign ia32e from 
> vmcs12->vm_exit_controls instead of vcpu->arch.efer

I agree with you. I was thinking do something like that but wasn't sure at all.

Best regards,
	Maxim Levitsky

> 
> Paolo
>