xen/balloon: Bring alloc(free)_xenballooned_pages helpers back

This patch rolls back some of the changes introduced by commit
121f2faca2c0a "xen/balloon: rename alloc/free_xenballooned_pages"
in order to make possible to still allocate xenballooned pages
if CONFIG_XEN_UNPOPULATED_ALLOC is enabled.

On Arm the unpopulated pages will be allocated on top of extended
regions provided by Xen via device-tree (the subsequent patches
will add required bits to support unpopulated-alloc feature on Arm).
The problem is that extended regions feature has been introduced
into Xen quite recently (during 4.16 release cycle). So this
effectively means that Linux must only use unpopulated-alloc on Arm
if it is running on "new Xen" which advertises these regions.
But, it will only be known after parsing the "hypervisor" node
at boot time, so before doing that we cannot assume anything.

In order to keep working if CONFIG_XEN_UNPOPULATED_ALLOC is enabled
and the extended regions are not advertised (Linux is running on
"old Xen", etc) we need the fallback to alloc_xenballooned_pages().

This way we wouldn't reduce the amount of memory usable (wasting
RAM pages) for any of the external mappings anymore (and eliminate
XSA-300) with "new Xen", but would be still functional ballooning
out RAM pages with "old Xen".

Also rename alloc(free)_xenballooned_pages to xen_alloc(free)_ballooned_pages
and make xen_alloc(free)_unpopulated_pages static inline in xen.h
if CONFIG_XEN_UNPOPULATED_ALLOC is disabled.

Signed-off-by: Oleksandr Tyshchenko <[e-mail]>
Reviewed-by: Stefano Stabellini <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Juergen Gross <[e-mail]>

drivers/xen/balloon.c

···
 }
 EXPORT_SYMBOL_GPL(balloon_set_new_target);
 
-#ifndef CONFIG_XEN_UNPOPULATED_ALLOC
 static int add_ballooned_pages(unsigned int nr_pages)
 {
 	enum bp_state st;
···
 }
 
 /**
- * xen_alloc_unpopulated_pages - get pages that have been ballooned out
+ * xen_alloc_ballooned_pages - get pages that have been ballooned out
  * @nr_pages: Number of pages to get
  * @pages: pages returned
  * @return 0 on success, error otherwise
  */
-int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages)
+int xen_alloc_ballooned_pages(unsigned int nr_pages, struct page **pages)
 {
 	unsigned int pgno = 0;
 	struct page *page;
···
 	return 0;
  out_undo:
 	mutex_unlock(&balloon_mutex);
-	xen_free_unpopulated_pages(pgno, pages);
+	xen_free_ballooned_pages(pgno, pages);
 	/*
-	 * NB: free_xenballooned_pages will only subtract pgno pages, but since
+	 * NB: xen_free_ballooned_pages will only subtract pgno pages, but since
 	 * target_unpopulated is incremented with nr_pages at the start we need
 	 * to remove the remaining ones also, or accounting will be screwed.
 	 */
 	balloon_stats.target_unpopulated -= nr_pages - pgno;
 	return ret;
 }
-EXPORT_SYMBOL(xen_alloc_unpopulated_pages);
+EXPORT_SYMBOL(xen_alloc_ballooned_pages);
 
 /**
- * xen_free_unpopulated_pages - return pages retrieved with get_ballooned_pages
+ * xen_free_ballooned_pages - return pages retrieved with get_ballooned_pages
  * @nr_pages: Number of pages
  * @pages: pages to return
  */
-void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages)
+void xen_free_ballooned_pages(unsigned int nr_pages, struct page **pages)
 {
 	unsigned int i;
 
···
 
 	mutex_unlock(&balloon_mutex);
 }
-EXPORT_SYMBOL(xen_free_unpopulated_pages);
+EXPORT_SYMBOL(xen_free_ballooned_pages);
 
-#if defined(CONFIG_XEN_PV)
+#if defined(CONFIG_XEN_PV) && !defined(CONFIG_XEN_UNPOPULATED_ALLOC)
 static void __init balloon_add_region(unsigned long start_pfn,
 				      unsigned long pages)
 {
···
 
 	balloon_stats.total_pages += extra_pfn_end - start_pfn;
 }
-#endif
 #endif
 
 static int __init balloon_init(void)

include/xen/balloon.h

···
 
 void balloon_set_new_target(unsigned long target);
 
+int xen_alloc_ballooned_pages(unsigned int nr_pages, struct page **pages);
+void xen_free_ballooned_pages(unsigned int nr_pages, struct page **pages);
+
 #ifdef CONFIG_XEN_BALLOON
 void xen_balloon_init(void);
 #else

include/xen/xen.h

···
 extern u64 xen_saved_max_mem_size;
 #endif
 
+#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
 int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages);
 void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages);
+#else
+#include <xen/balloon.h>
+static inline int xen_alloc_unpopulated_pages(unsigned int nr_pages,
+		struct page **pages)
+{
+	return xen_alloc_ballooned_pages(nr_pages, pages);
+}
+static inline void xen_free_unpopulated_pages(unsigned int nr_pages,
+		struct page **pages)
+{
+	xen_free_ballooned_pages(nr_pages, pages);
+}
+#endif
 
 #endif	/* _XEN_XEN_H */