powerpc/mm: Add pte_xchg() helper

We have five locations in 64-bit hash MMU code that do a cmpxchg() of a
PTE. Currently doing it inline OK, but in a future patch we will be
converting the PTEs to __be64 in some configs. In that case we will need
casts at every cmpxchg() site in order to keep sparse happy.

So move the logic into a helper, this is a reasonably nice cleanup on
its own.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/kvm_book3s_64.h

@@ -319,11 +319,8 @@ static inline pte_t kvmppc_read_update_linux_pte(pte_t *ptep, int writing)
 		if (writing && pte_write(old_pte))
 			new_pte = pte_mkdirty(new_pte);
 
-		if (pte_val(old_pte) == __cmpxchg_u64((unsigned long *)ptep,
-						      pte_val(old_pte),
-						      pte_val(new_pte))) {
+		if (pte_xchg(ptep, old_pte, new_pte))
 			break;
-		}
 	}
 	return new_pte;
 }

arch/powerpc/include/asm/pgtable-types.h

@@ -54,4 +54,16 @@ typedef struct { pte_t pte; unsigned long hidx; } real_pte_t;
 #else
 typedef struct { pte_t pte; } real_pte_t;
 #endif
+
+#ifdef CONFIG_PPC_STD_MMU_64
+#include <asm/cmpxchg.h>
+
+static inline bool pte_xchg(pte_t *ptep, pte_t old, pte_t new)
+{
+	unsigned long *p = (unsigned long *)ptep;
+
+	return pte_val(old) == __cmpxchg_u64(p, pte_val(old), pte_val(new));
+}
+#endif
+
 #endif /* _ASM_POWERPC_PGTABLE_TYPES_H */