... and use it where suitable (the tmem caller doesn't propagate an
error code). While it doesn't matter as much, also make donate_page()
follow suit on x86 (on ARM it already returns -ENOSYS).

Also move their declarations to common code and add __must_check.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -1090,7 +1090,7 @@ int donate_page(struct domain *d, struct
 int steal_page(
     struct domain *d, struct page_info *page, unsigned int memflags)
 {
-    return -1;
+    return -EOPNOTSUPP;
 }
 
 int page_is_ram_type(unsigned long mfn, unsigned long mem_type)
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4424,7 +4424,7 @@ int donate_page(
              page_to_mfn(page), d->domain_id,
              owner ? owner->domain_id : DOMID_INVALID,
              page->count_info, page->u.inuse.type_info);
-    return -1;
+    return -EINVAL;
 }
 
 int steal_page(
@@ -4435,7 +4435,7 @@ int steal_page(
     const struct domain *owner = dom_xen;
 
     if ( paging_mode_external(d) )
-        return -1;
+        return -EOPNOTSUPP;
 
     spin_lock(&d->page_alloc_lock);
 
@@ -4490,7 +4490,7 @@ int steal_page(
              page_to_mfn(page), d->domain_id,
              owner ? owner->domain_id : DOMID_INVALID,
              page->count_info, page->u.inuse.type_info);
-    return -1;
+    return -EINVAL;
 }
 
 static int __do_update_va_mapping(
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -1843,10 +1843,10 @@ gnttab_transfer(
         }
 
         page = mfn_to_page(mfn);
-        if ( steal_page(d, page, 0) < 0 )
+        if ( (rc = steal_page(d, page, 0)) < 0 )
         {
             put_gfn(d, gop.mfn);
-            gop.status = GNTST_bad_page;
+            gop.status = rc == -EINVAL ? GNTST_bad_page : GNTST_general_error;
             goto copyback;
         }
 
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -566,10 +566,10 @@ static long memory_exchange(XEN_GUEST_HA
 
                 page = mfn_to_page(mfn);
 
-                if ( unlikely(steal_page(d, page, MEMF_no_refcount)) )
+                rc = steal_page(d, page, MEMF_no_refcount);
+                if ( unlikely(rc) )
                 {
                     put_gfn(d, gmfn + k);
-                    rc = -EINVAL;
                     goto fail;
                 }
 
--- a/xen/include/asm-arm/mm.h
+++ b/xen/include/asm-arm/mm.h
@@ -322,11 +322,6 @@ static inline int relinquish_shared_page
 /* Arch-specific portion of memory_op hypercall. */
 long arch_memory_op(int op, XEN_GUEST_HANDLE_PARAM(void) arg);
 
-int steal_page(
-    struct domain *d, struct page_info *page, unsigned int memflags);
-int donate_page(
-    struct domain *d, struct page_info *page, unsigned int memflags);
-
 #define domain_set_alloc_bitsize(d) ((void)0)
 #define domain_clamp_alloc_bitsize(d, b) (b)
 
--- a/xen/include/asm-x86/mm.h
+++ b/xen/include/asm-x86/mm.h
@@ -550,11 +550,6 @@ long subarch_memory_op(unsigned long cmd
 int compat_arch_memory_op(unsigned long cmd, XEN_GUEST_HANDLE_PARAM(void));
 int compat_subarch_memory_op(int op, XEN_GUEST_HANDLE_PARAM(void));
 
-int steal_page(
-    struct domain *d, struct page_info *page, unsigned int memflags);
-int donate_page(
-    struct domain *d, struct page_info *page, unsigned int memflags);
-
 int map_ldt_shadow_page(unsigned int);
 
 #define NIL(type) ((type *)-sizeof(type))
--- a/xen/include/xen/mm.h
+++ b/xen/include/xen/mm.h
@@ -567,8 +567,12 @@ int xenmem_add_to_physmap_one(struct dom
                               union xen_add_to_physmap_batch_extra extra,
                               unsigned long idx, gfn_t gfn);
 
-/* Returns 0 on success, or negative on error. */
+/* Return 0 on success, or negative on error. */
 int __must_check guest_remove_page(struct domain *d, unsigned long gmfn);
+int __must_check steal_page(struct domain *d, struct page_info *page,
+                            unsigned int memflags);
+int __must_check donate_page(struct domain *d, struct page_info *page,
+                             unsigned int memflags);
 
 #define RAM_TYPE_CONVENTIONAL 0x00000001
 #define RAM_TYPE_RESERVED     0x00000002