Re: KVM: VMX: disable SMEP feature when guest is in non-paging mode

[Gleb Natapov <gleb@redhat.com>]

On Fri, Feb 01, 2013 at 08:30:07AM -0000, Xu wrote:
> SMEP is disabled if CPU is in non-paging mode in hardware.
> However KVM always uses paging mode to emulate guest non-paging
> mode with HAP.
Not always, only if unrestricted mode is disabled, since vm86 mode, that
is used otherwise, requires paging.

>                To emulate this behavior, SMEP needs to be manually
> disabled when guest switches to non-paging mode.
> 
> We met an issue that, SMP Linux guest with recent kernel (enable
> SMEP support, for example, 3.5.3) would crash with triple fault if
> setting unrestricted_guest=0. This is because KVM uses an identity
> mapping page table to emulate the non-paging mode, where the page
> table is set with USER flag. If SMEP is still enabled in this case,
> guest will meet unhandlable page fault and then crash.
> 
> Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
> Signed-off-by: Xiantao Zhang <xiantao.zhang@intel.com>
Reviewed-by: Gleb Natapov <gleb@redhat.com>

But HAP is XEN terminology AFAIK. KVM speak is tdp (two dimensional
paging). If would be nice to change it in the commit message and the
comment before committing.

> 
> ---
> arch/x86/kvm/vmx.c |    8 ++++++++
>  1 files changed, 8 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 9120ae1..e82f20d 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -3155,6 +3155,14 @@ static int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>  		if (!is_paging(vcpu)) {
>  			hw_cr4 &= ~X86_CR4_PAE;
>  			hw_cr4 |= X86_CR4_PSE;
> +			/*
> +			 * SMEP is disabled if CPU is in non-paging mode in
> +			 * hardware. However KVM always uses paging mode to
> +			 * emulate guest non-paging mode with HAP.
> +			 * To emulate this behavior, SMEP needs to be manually
> +			 * disabled when guest switches to non-paging mode.
> +			 */
> +			hw_cr4 &= ~X86_CR4_SMEP;
>  		} else if (!(cr4 & X86_CR4_PAE)) {
>  			hw_cr4 &= ~X86_CR4_PAE;
>  		}

--
			Gleb.