VMX: Always disable SMEP when guest is in non-paging mode

commit e7dda8ec9f
  VMX: disable SMEP feature when guest is in non-paging mode

disabled the SMEP bit if a guest VCPU was using HAP and was not
in paging mode. However I could observe VCPUs getting stuck in
the trampoline after the following patch in the Linux kernel
changed the way CR4 gets set up:
  x86, realmode: read cr4 and EFER from kernel for 64-bit trampoline

The change will set CR4 from already set flags which includes the
SMEP bit. On bare metal this does not matter as the CPU is in non-
paging mode at that time. But Xen seems to use the emulated non-
paging mode regardless of HAP (I verified that on the guests I was
seeing the issue, HAP was not used).

Therefor it seems right to unset the SMEP bit for a VCPU that is
not in paging-mode, regardless of its HAP usage.

Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Keir Fraser <keir@xen.org>
Acked-by: Dongxiao Xu <dongxiao.xu@intel.com>

xen/arch/x86/hvm/vmx/vmx.c

@@ -1162,11 +1162,14 @@ static void vmx_update_guest_cr(struct vcpu *v, unsigned int cr)
         {
             v->arch.hvm_vcpu.hw_cr[4] |= X86_CR4_PSE;
             v->arch.hvm_vcpu.hw_cr[4] &= ~X86_CR4_PAE;
+        }
+        if ( !hvm_paging_enabled(v) )
+        {
             /*
              * SMEP is disabled if CPU is in non-paging mode in hardware.
              * However Xen 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.
+             * mode. To emulate this behavior, SMEP needs to be manually
+             * disabled when guest VCPU is in non-paging mode.
              */
             v->arch.hvm_vcpu.hw_cr[4] &= ~X86_CR4_SMEP;
         }