xen: introduce helper functions to do safe read and write accesses

Introduce two helper functions to safely read and write unsigned long
values from or to memory when the access may fault because the mapping
is non-present or read-only.

These helpers can be used instead of open coded uses of __get_user()
and __put_user() avoiding the need to do casts to fix sparse warnings.

Use the helpers in page.h and p2m.c. This will fix the sparse
warnings when doing "make C=1".
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

arch/x86/include/asm/xen/page.h

@@ -59,6 +59,20 @@ extern int clear_foreign_p2m_mapping(struct gnttab_unmap_grant_ref *unmap_ops,
 				     struct page **pages, unsigned int count);
 extern unsigned long m2p_find_override_pfn(unsigned long mfn, unsigned long pfn);
 
+/*
+ * Helper functions to write or read unsigned long values to/from
+ * memory, when the access may fault.
+ */
+static inline int xen_safe_write_ulong(unsigned long *addr, unsigned long val)
+{
+	return __put_user(val, (unsigned long __user *)addr);
+}
+
+static inline int xen_safe_read_ulong(unsigned long *addr, unsigned long *val)
+{
+	return __get_user(*val, (unsigned long __user *)addr);
+}
+
 /*
  * When to use pfn_to_mfn(), __pfn_to_mfn() or get_phys_to_machine():
  * - pfn_to_mfn() returns either INVALID_P2M_ENTRY or the mfn. No indicator
@@ -125,7 +139,7 @@ static inline unsigned long mfn_to_pfn_no_overrides(unsigned long mfn)
 	 * In such cases it doesn't matter what we return (we return garbage),
 	 * but we must handle the fault without crashing!
 	 */
-	ret = __get_user(pfn, &machine_to_phys_mapping[mfn]);
+	ret = xen_safe_read_ulong(&machine_to_phys_mapping[mfn], &pfn);
 	if (ret < 0)
 		return ~0;
 

arch/x86/xen/p2m.c

@@ -625,7 +625,7 @@ bool __set_phys_to_machine(unsigned long pfn, unsigned long mfn)
 		return true;
 	}
 
-	if (likely(!__put_user(mfn, xen_p2m_addr + pfn)))
+	if (likely(!xen_safe_write_ulong(xen_p2m_addr + pfn, mfn)))
 		return true;
 
 	ptep = lookup_address((unsigned long)(xen_p2m_addr + pfn), &level);