Re: [PATCH KVM VMX 2/2] KVM: VMX: Support Unrestricted Guest feature

[Avi Kivity <avi@redhat.com>]

Nitin A Kamble wrote:
> "Unrestricted Guest" feature is added in the VMX specification.
> Intel Westmere and onwards processors will support this feature.
>
>     It allows kvm guests to run real mode and unpaged mode
> code natively in the VMX mode when EPT is turned on. With the
> unrestricted guest there is no need to emulate the guest real mode code
> in the vm86 container or in the emulator. Also the guest big real mode
> code works like native.
>
>   The attached patch enhances KVM to use the unrestricted guest feature
> if available on the processor. It also adds a new kernel/module
> parameter to disable the unrestricted guest feature at the boot time.
>
>  
> @@ -1431,6 +1447,9 @@ static void enter_rmode(struct kvm_vcpu *vcpu)
>  	unsigned long flags;
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
>  
> +	if (enable_unrestricted_guest)
> +		goto continue_rmode;
> +
>   

I think you should just return here.  You don't need to reset the mmu 
context or run the init_rmode() code.  Maybe you need to flush the tlb.



> +
> +	if (enable_unrestricted_guest && (seg != VCPU_SREG_LDTR))
> +		ar |= 0x1; /* Accessed */
> +
>  	vmcs_write32(sf->ar_bytes, ar);
>  }
>   

Please add a comment indicating it's for broken userspace that doesn't 
set the A bit.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.