[PATCH v5 1/6] 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 v5:
- Update the comment above update_intpte()

Changes in v4:
- Remove UPDATE_ENTRY_SWAP
- Don't move the backslashes in UPDATE_ENTRY

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 | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/pv/mm.h b/xen/arch/x86/pv/mm.h
index bfee0feb7b21..4016dd1087ba 100644
--- a/xen/arch/x86/pv/mm.h
+++ b/xen/arch/x86/pv/mm.h
@@ -64,16 +64,22 @@ 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.
  */
-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
     {
@@ -95,6 +101,8 @@ static inline void update_intpte(intpte_t *p, intpte_t old, 
intpte_t new,
             old = t;
         }
     }
+
+    return old;
 }
 
 /*
-- 
2.52.0