Re: [PATCH] KVM: nVMX: mask unrestricted_guest if disabled on L0

[Kashyap Chamarthy <kchamart@redhat.com>]

On Tue, Feb 24, 2015 at 05:30:06PM +0100, Radim Krčmář wrote:
> 2015-02-23 19:05+0100, Kashyap Chamarthy:
> > Tested with the _correct_ Kernel[1] (that has Radim's patch) now --
> > applied it on both L0 and L1.
> > 
> > Result: Same as before -- Booting L2 causes L1 to reboot. However, the
> >         stack trace from `dmesg` on L0 is took slightly different path than
> >         before -- it's using MSR handling:
> 
> Thanks, the problem was deeper ... L1 enabled unrestricted mode while L0
> had it disabled.  L1 could then vmrun a L2 state that L0 would have to
> emulate, but that doesn't work.  There are at least these solutions:
> 
>  1) don't expose unrestricted_guest when L0 doesn't have it
>  2) fix unrestricted mode emulation code
>  3) handle the failure a without killing L1
> 
> I'd do just (1) -- emulating unrestricted mode is a loss.
> 
> I have done initial testing and at least qemu-sanity-check works now:
> 
> ---8<---
> If EPT was enabled, unrestricted_guest was allowed in L1 regardless of
> L0.  L1 triple faulted when running L2 guest that required emulation.
> 
> Another side effect was 'WARN_ON_ONCE(vmx->nested.nested_run_pending)'
> in L0's dmesg:
>   WARNING: CPU: 0 PID: 0 at arch/x86/kvm/vmx.c:9190 nested_vmx_vmexit+0x96e/0xb00 [kvm_intel] ()
> 
> Prevent this scenario by masking SECONDARY_EXEC_UNRESTRICTED_GUEST when
> the host doesn't have it enabled.
> 
> Fixes: 78051e3b7e35 ("KVM: nVMX: Disable unrestricted mode if ept=0")
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>


I just built[1] a Kernel with this patch and tested it on L0 and L1 and
can confirm, the patch fixes the issue -- Booting L2 does not cause L1
to reboot.

So:

    Tested-By: Kashyap Chamarthy <kchamart@redhat.com>

Thanks for investigating, Radim!

[1] https://kashyapc.fedorapeople.org/kernel-4.0.0-0.rc1.git1.1.kashyap1.fc23-with-nvmx-fix2-radim/


> ---
>  arch/x86/kvm/vmx.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index f7b20b417a3a..dbabea21357b 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -2476,8 +2476,7 @@ static void nested_vmx_setup_ctls_msrs(struct vcpu_vmx *vmx)
>  	if (enable_ept) {
>  		/* nested EPT: emulate EPT also to L1 */
>  		vmx->nested.nested_vmx_secondary_ctls_high |=
> -			SECONDARY_EXEC_ENABLE_EPT |
> -			SECONDARY_EXEC_UNRESTRICTED_GUEST;
> +			SECONDARY_EXEC_ENABLE_EPT;
>  		vmx->nested.nested_vmx_ept_caps = VMX_EPT_PAGE_WALK_4_BIT |
>  			 VMX_EPTP_WB_BIT | VMX_EPT_2MB_PAGE_BIT |
>  			 VMX_EPT_INVEPT_BIT;
> @@ -2491,6 +2490,10 @@ static void nested_vmx_setup_ctls_msrs(struct vcpu_vmx *vmx)
>  	} else
>  		vmx->nested.nested_vmx_ept_caps = 0;
>  
> +	if (enable_unrestricted_guest)
> +		vmx->nested.nested_vmx_secondary_ctls_high |=
> +			SECONDARY_EXEC_UNRESTRICTED_GUEST;
> +
>  	/* miscellaneous data */
>  	rdmsr(MSR_IA32_VMX_MISC,
>  		vmx->nested.nested_vmx_misc_low,

-- 
/kashyap