[PATCH 2/2] x86/mm: remove some indirection from {paging,sh}_cmpxchg_guest_entry()

[Jan Beulich <jbeulich@xxxxxxxx>]

Make the functions more similar to cmpxchg() in that they now take an
integral "old" input and return the value read.

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

--- a/xen/arch/x86/mm/shadow/private.h
+++ b/xen/arch/x86/mm/shadow/private.h
@@ -398,8 +398,8 @@ int shadow_write_p2m_entry(struct p2m_do
 /* Functions that atomically write PV guest PT entries */
 void sh_write_guest_entry(struct vcpu *v, intpte_t *p, intpte_t new,
                           mfn_t gmfn);
-void sh_cmpxchg_guest_entry(struct vcpu *v, intpte_t *p, intpte_t *old,
-                            intpte_t new, mfn_t gmfn);
+intpte_t sh_cmpxchg_guest_entry(struct vcpu *v, intpte_t *p, intpte_t old,
+                                intpte_t new, mfn_t gmfn);
 
 /* Update all the things that are derived from the guest's CR0/CR3/CR4.
  * Called to initialize paging structures if the paging mode
--- a/xen/arch/x86/mm/shadow/pv.c
+++ b/xen/arch/x86/mm/shadow/pv.c
@@ -39,22 +39,22 @@ sh_write_guest_entry(struct vcpu *v, int
 
 /*
  * Cmpxchg a new value into the guest pagetable, and update the shadows
- * appropriately.
- * N.B. caller should check the value of "old" to see if the cmpxchg itself
- * was successful.
+ * appropriately.  Returns the previous entry found, which the caller is
+ * expected to check to see if the cmpxchg was successful.
  */
-void
-sh_cmpxchg_guest_entry(struct vcpu *v, intpte_t *p, intpte_t *old,
+intpte_t
+sh_cmpxchg_guest_entry(struct vcpu *v, intpte_t *p, intpte_t old,
                        intpte_t new, mfn_t gmfn)
 {
     intpte_t t;
 
     paging_lock(v->domain);
-    t = cmpxchg(p, *old, new);
-    if ( t == *old )
+    t = cmpxchg(p, old, new);
+    if ( t == old )
         sh_validate_guest_entry(v, gmfn, p, sizeof(new));
-    *old = t;
     paging_unlock(v->domain);
+
+    return t;
 }
 
 /*
--- a/xen/arch/x86/pv/mm.h
+++ b/xen/arch/x86/pv/mm.h
@@ -47,16 +47,14 @@ static inline bool update_intpte(intpte_
     else
 #endif
     {
-        intpte_t t = old;
-
         for ( ; ; )
         {
-            intpte_t _new = new;
+            intpte_t _new = new, t;
 
             if ( preserve_ad )
                 _new |= old & (_PAGE_ACCESSED | _PAGE_DIRTY);
 
-            paging_cmpxchg_guest_entry(v, p, &t, _new, mfn);
+            t = paging_cmpxchg_guest_entry(v, p, old, _new, mfn);
 
             if ( t == old )
                 break;
--- a/xen/arch/x86/pv/ro-page-fault.c
+++ b/xen/arch/x86/pv/ro-page-fault.c
@@ -168,8 +168,8 @@ static int ptwr_emulated_update(unsigned
     if ( p_old )
     {
         ol1e = l1e_from_intpte(old);
-        paging_cmpxchg_guest_entry(v, &l1e_get_intpte(*pl1e), &old,
-                                   l1e_get_intpte(nl1e), mfn);
+        old = paging_cmpxchg_guest_entry(v, &l1e_get_intpte(*pl1e), old,
+                                         l1e_get_intpte(nl1e), mfn);
         if ( l1e_get_intpte(ol1e) == old )
             ret = X86EMUL_OKAY;
         else
--- a/xen/include/asm-x86/paging.h
+++ b/xen/include/asm-x86/paging.h
@@ -98,8 +98,8 @@ struct shadow_paging_mode {
 #ifdef CONFIG_PV
     void          (*write_guest_entry     )(struct vcpu *v, intpte_t *p,
                                             intpte_t new, mfn_t gmfn);
-    void          (*cmpxchg_guest_entry   )(struct vcpu *v, intpte_t *p,
-                                            intpte_t *old, intpte_t new,
+    intpte_t      (*cmpxchg_guest_entry   )(struct vcpu *v, intpte_t *p,
+                                            intpte_t old, intpte_t new,
                                             mfn_t gmfn);
 #endif
 #ifdef CONFIG_HVM
@@ -342,16 +342,15 @@ static inline void paging_write_guest_en
  * true if not.  N.B. caller should check the value of "old" to see if the
  * cmpxchg itself was successful.
  */
-static inline void paging_cmpxchg_guest_entry(
-    struct vcpu *v, intpte_t *p, intpte_t *old, intpte_t new, mfn_t gmfn)
+static inline intpte_t paging_cmpxchg_guest_entry(
+    struct vcpu *v, intpte_t *p, intpte_t old, intpte_t new, mfn_t gmfn)
 {
 #ifdef CONFIG_SHADOW_PAGING
     if ( unlikely(paging_mode_shadow(v->domain)) && paging_get_hostmode(v) )
-        paging_get_hostmode(v)->shadow.cmpxchg_guest_entry(v, p, old,
-                                                           new, gmfn);
-    else
+        return paging_get_hostmode(v)->shadow.cmpxchg_guest_entry(v, p, old,
+                                                                  new, gmfn);
 #endif
-        *old = cmpxchg(p, *old, new);
+    return cmpxchg(p, old, new);
 }
 
 #endif /* CONFIG_PV */