[PATCH v3 2/6] x86: extend update_intpte() to support atomic get-and-update

[Kevin Lampis <kevin.lampis@xxxxxxxxxx>]

The update_intpte() now accepts a new swap flag and if set
returns the old pte value.

No functional change for existing callers.

Signed-off-by: Kevin Lampis <kevin.lampis@xxxxxxxxxx>
---
Changes in v3:
- Return 0 in the paging_write_guest_entry() case (update_intpte)
- Add UPDATE_ENTRY_SWAP to convert return value to correct type

Changes in v2:
- Add new PTE_UPDATE_SWAP flag instead of a bool argument
- Return the old pte value instead of turning `old` into an out pointer
---
 xen/arch/x86/pv/mm.h | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/pv/mm.h b/xen/arch/x86/pv/mm.h
index bfee0feb7b21..363db35b5fe3 100644
--- a/xen/arch/x86/pv/mm.h
+++ b/xen/arch/x86/pv/mm.h
@@ -64,16 +64,23 @@ static inline intpte_t paging_cmpxchg_guest_entry(
 
 #define PTE_UPDATE_PRESERVE_AD  (1u << 0)
 #define PTE_UPDATE_NO_TRANSLATE (1u << 1)
+#define PTE_UPDATE_SWAP         (1u << 2)
 
 /*
  * How to write an entry to the guest pagetables.
+ * If the SWAP flag is set then does an atomic swap and returns the old PTE
+ * value, otherwise returns 0.
  */
-static inline void update_intpte(intpte_t *p, intpte_t old, intpte_t new,
-                                 mfn_t mfn, struct vcpu *v, unsigned int flags)
+static inline intpte_t update_intpte(intpte_t *p, intpte_t old, intpte_t new,
+                                     mfn_t mfn, struct vcpu *v,
+                                     unsigned int flags)
 {
 #ifndef PTE_UPDATE_WITH_CMPXCHG
-    if ( !(flags & PTE_UPDATE_PRESERVE_AD) )
+    if ( !(flags & (PTE_UPDATE_PRESERVE_AD | PTE_UPDATE_SWAP)) )
+    {
         paging_write_guest_entry(v, p, new, mfn);
+        return 0;
+    }
     else
 #endif
     {
@@ -95,17 +102,26 @@ static inline void update_intpte(intpte_t *p, intpte_t 
old, intpte_t new,
             old = t;
         }
     }
+    return old;
 }
 
 /*
  * 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 , fl)                   \
-    update_intpte(&_t ## e_get_intpte(*(_p)),                       \
-                  _t ## e_get_intpte(_o), _t ## e_get_intpte(_n),   \
+#define UPDATE_ENTRY(_t ,_p ,_o ,_n ,_m ,_v , fl)                 \
+    update_intpte(&_t ## e_get_intpte(*(_p)),                     \
+                  _t ## e_get_intpte(_o), _t ## e_get_intpte(_n), \
                   _m, _v, fl)
 
+/*
+ * Macro that wraps UPDATE_ENTRY.
+ * This version converts the return type to l{1,2,3,4}_pgentry_t
+ * The return value must be used because of compiler flags.
+ */
+#define UPDATE_ENTRY_SWAP(_t ,_p ,_o ,_n ,_m ,_v , fl)            \
+    _t ## e_from_intpte(UPDATE_ENTRY(_t, _p, _o, _n, _m, _v, fl))
+
 static always_inline l1_pgentry_t adjust_guest_l1e(l1_pgentry_t l1e,
                                                    const struct domain *d)
 {
-- 
2.52.0