VMX: fix PAT value seen by guest

The XSA-60 fixes introduced a window during which the guest PAT gets
forced to all zeros. This shouldn't be visible to the guest. Therefore
we need to intercept PAT MSR accesses during that time period.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -1033,6 +1033,8 @@ static void vmx_handle_cd(struct vcpu *v
 
             vmx_get_guest_pat(v, pat);
             vmx_set_guest_pat(v, uc_pat);
+            vmx_enable_intercept_for_msr(v, MSR_IA32_CR_PAT,
+                                         MSR_TYPE_R | MSR_TYPE_W);
 
             wbinvd();               /* flush possibly polluted cache */
             hvm_asid_flush_vcpu(v); /* invalidate memory type cached in TLB */
@@ -1042,6 +1044,9 @@ static void vmx_handle_cd(struct vcpu *v
         {
             v->arch.hvm_vcpu.cache_mode = NORMAL_CACHE_MODE;
             vmx_set_guest_pat(v, *pat);
+            if ( !iommu_enabled || iommu_snoop )
+                vmx_disable_intercept_for_msr(v, MSR_IA32_CR_PAT,
+                                              MSR_TYPE_R | MSR_TYPE_W);
             hvm_asid_flush_vcpu(v); /* no need to flush cache */
         }
     }