[PATCH] VT-x: simplify/clarify vmx_load_pdptrs()

[Jan Beulich <jbeulich@xxxxxxxx>]

* Guests outside of long mode can't have PCID enabled. Drop the
  respective check to make more obvious that there's no security issue
  (from potentially accessing past the mapped page's boundary).

* Only the low 32 bits of CR3 are relevant outside of long mode, even
  if they remained unchanged after leaving that mode.

* Drop the unnecessary and badly typed local variable p.

* Don't open-code hvm_long_mode_active() (and extend this to the related
  nested VT-x code).

* Constify guest_pdptes to clarify that we're only reading from the
  page.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
This is intentionally not addressing any of the other shortcomings of
the function, as was done by the previously posted
https://lists.xenproject.org/archives/html/xen-devel/2018-07/msg01790.html.
This will need to be the subject of a further change.

--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -1312,17 +1312,16 @@ static void vmx_set_interrupt_shadow(str
 
 static void vmx_load_pdptrs(struct vcpu *v)
 {
-    unsigned long cr3 = v->arch.hvm.guest_cr[3];
-    uint64_t *guest_pdptes;
+    uint32_t cr3 = v->arch.hvm.guest_cr[3];
+    const uint64_t *guest_pdptes;
     struct page_info *page;
     p2m_type_t p2mt;
-    char *p;
 
     /* EPT needs to load PDPTRS into VMCS for PAE. */
-    if ( !hvm_pae_enabled(v) || (v->arch.hvm.guest_efer & EFER_LMA) )
+    if ( !hvm_pae_enabled(v) || hvm_long_mode_active(v) )
         return;
 
-    if ( (cr3 & 0x1fUL) && !hvm_pcid_enabled(v) )
+    if ( cr3 & 0x1f )
         goto crash;
 
     page = get_page_from_gfn(v->domain, cr3 >> PAGE_SHIFT, &p2mt, P2M_UNSHARE);
@@ -1332,14 +1331,12 @@ static void vmx_load_pdptrs(struct vcpu
          * queue, but this is the wrong place. We're holding at least
          * the paging lock */
         gdprintk(XENLOG_ERR,
-                 "Bad cr3 on load pdptrs gfn %lx type %d\n",
+                 "Bad cr3 on load pdptrs gfn %"PRIx32" type %d\n",
                  cr3 >> PAGE_SHIFT, (int) p2mt);
         goto crash;
     }
 
-    p = __map_domain_page(page);
-
-    guest_pdptes = (uint64_t *)(p + (cr3 & ~PAGE_MASK));
+    guest_pdptes = __map_domain_page(page) + (cr3 & ~PAGE_MASK);
 
     /*
      * We do not check the PDPTRs for validity. The CPU will do this during
@@ -1356,7 +1353,7 @@ static void vmx_load_pdptrs(struct vcpu
 
     vmx_vmcs_exit(v);
 
-    unmap_domain_page(p);
+    unmap_domain_page(guest_pdptes);
     put_page(page);
     return;
 
--- a/xen/arch/x86/hvm/vmx/vvmx.c
+++ b/xen/arch/x86/hvm/vmx/vvmx.c
@@ -1234,7 +1234,7 @@ static void virtual_vmentry(struct cpu_u
         paging_update_paging_modes(v);
 
     if ( nvmx_ept_enabled(v) && hvm_pae_enabled(v) &&
-         !(v->arch.hvm.guest_efer & EFER_LMA) )
+         !hvm_long_mode_active(v) )
         vvmcs_to_shadow_bulk(v, ARRAY_SIZE(gpdpte_fields), gpdpte_fields);
 
     regs->rip = get_vvmcs(v, GUEST_RIP);
@@ -1437,7 +1437,7 @@ static void virtual_vmexit(struct cpu_us
     sync_exception_state(v);
 
     if ( nvmx_ept_enabled(v) && hvm_pae_enabled(v) &&
-         !(v->arch.hvm.guest_efer & EFER_LMA) )
+         !hvm_long_mode_active(v) )
         shadow_to_vvmcs_bulk(v, ARRAY_SIZE(gpdpte_fields), gpdpte_fields);
 
     /* This will clear current pCPU bit in p2m->dirty_cpumask */