[PATCH 1/4] x86: extend update_intpte() to support atomic get-and-update

[Kevin Lampis <kevin.lampis@xxxxxxxxxx>]

The update_intpte() function now accepts a new use_cmpxchg flag and if set
returns the old pte value through the new *old pointer.

No functional change for existing callers.

Signed-off-by: Kevin Lampis <kevin.lampis@xxxxxxxxxx>
---
 xen/arch/x86/pv/mm.h | 29 ++++++++++++++++++-----------
 1 file changed, 18 insertions(+), 11 deletions(-)

diff --git a/xen/arch/x86/pv/mm.h b/xen/arch/x86/pv/mm.h
index 4564cab9fc0f..8c42cf3b3f4d 100644
--- a/xen/arch/x86/pv/mm.h
+++ b/xen/arch/x86/pv/mm.h
@@ -66,13 +66,14 @@ static inline intpte_t paging_cmpxchg_guest_entry(
  * How to write an entry to the guest pagetables.
  * Returns false for failure (pointer not valid), true for success.
  */
-static inline bool update_intpte(intpte_t *p, intpte_t old, intpte_t new,
-                                 mfn_t mfn, struct vcpu *v, bool preserve_ad)
+static inline bool update_intpte(intpte_t *p, intpte_t *old, intpte_t new,
+                                 mfn_t mfn, struct vcpu *v, bool preserve_ad,
+                                 bool use_cmpxchg)
 {
     bool rv = true;
 
 #ifndef PTE_UPDATE_WITH_CMPXCHG
-    if ( !preserve_ad )
+    if ( !preserve_ad && !use_cmpxchg )
         paging_write_guest_entry(v, p, new, mfn);
     else
 #endif
@@ -82,30 +83,36 @@ static inline bool update_intpte(intpte_t *p, intpte_t old, 
intpte_t new,
             intpte_t _new = new, t;
 
             if ( preserve_ad )
-                _new |= old & (_PAGE_ACCESSED | _PAGE_DIRTY);
+                _new |= *old & (_PAGE_ACCESSED | _PAGE_DIRTY);
 
-            t = paging_cmpxchg_guest_entry(v, p, old, _new, mfn);
+            t = paging_cmpxchg_guest_entry(v, p, *old, _new, mfn);
 
-            if ( t == old )
+            if ( t == *old )
                 break;
 
             /* Allowed to change in Accessed/Dirty flags only. */
-            BUG_ON((t ^ old) & ~(intpte_t)(_PAGE_ACCESSED|_PAGE_DIRTY));
+            BUG_ON((t ^ *old) & ~(intpte_t)(_PAGE_ACCESSED|_PAGE_DIRTY));
 
-            old = t;
+            *old = t;
         }
     }
     return rv;
 }
 
+static inline bool _update_intpte(intpte_t *p, intpte_t old, intpte_t new,
+                                  mfn_t mfn, struct vcpu *v, bool preserve_ad)
+{
+    return update_intpte(p, &old, new, mfn, v, preserve_ad, false);
+}
+
 /*
  * Macro that wraps the appropriate type-changes around update_intpte().
  * Arguments are: type, ptr, old, new, mfn, vcpu
  */
 #define UPDATE_ENTRY(_t,_p,_o,_n,_m,_v,_ad)                         \
-    update_intpte(&_t ## e_get_intpte(*(_p)),                       \
-                  _t ## e_get_intpte(_o), _t ## e_get_intpte(_n),   \
-                  (_m), (_v), (_ad))
+    _update_intpte(&_t ## e_get_intpte(*(_p)),                      \
+                   _t ## e_get_intpte(_o), _t ## e_get_intpte(_n),  \
+                   (_m), (_v), (_ad))
 
 static always_inline l1_pgentry_t adjust_guest_l1e(l1_pgentry_t l1e,
                                                    const struct domain *d)
-- 
2.52.0