xen/balloon: Seperate the auto-translate logic properly (v2)

The auto-xlat logic vs the non-xlat means that we don't need to for
auto-xlat guests (like PVH, HVM or ARM):
 - use P2M
 - use scratch page.

However the code in increase_reservation does modify the p2m for
auto_translate guests, but not in decrease_reservation.

Fix that by avoiding any p2m modifications in both increase_reservation
and decrease_reservation for auto_translated guests.

And also avoid allocating or using scratch pages for auto_translated guests.

Lastly, since !auto-xlat is really another way of saying 'xen_pv'
remove the redundant 'xen_pv_domain' check.
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
[v2: Updated the description]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/balloon.c

@@ -350,11 +350,12 @@ static enum bp_state increase_reservation(unsigned long nr_pages)
 
 		pfn = page_to_pfn(page);
 
+#ifdef CONFIG_XEN_HAVE_PVMMU
+		if (!xen_feature(XENFEAT_auto_translated_physmap)) {
 			set_phys_to_machine(pfn, frame_list[i]);
 
-#ifdef CONFIG_XEN_HAVE_PVMMU
 			/* Link back into the page tables if not highmem. */
-		if (xen_pv_domain() && !PageHighMem(page)) {
+			if (!PageHighMem(page)) {
 				int ret;
 				ret = HYPERVISOR_update_va_mapping(
 						(unsigned long)__va(pfn << PAGE_SHIFT),
@@ -362,6 +363,7 @@ static enum bp_state increase_reservation(unsigned long nr_pages)
 						0);
 				BUG_ON(ret);
 			}
+		}
 #endif
 
 		/* Relinquish the page back to the allocator. */
@@ -378,7 +380,6 @@ static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
 	enum bp_state state = BP_DONE;
 	unsigned long  pfn, i;
 	struct page   *page;
-	struct page   *scratch_page;
 	int ret;
 	struct xen_memory_reservation reservation = {
 		.address_bits = 0,
@@ -411,27 +412,29 @@ static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
 
 		scrub_page(page);
 
+#ifdef CONFIG_XEN_HAVE_PVMMU
 		/*
 		 * Ballooned out frames are effectively replaced with
 		 * a scratch frame.  Ensure direct mappings and the
 		 * p2m are consistent.
 		 */
-		scratch_page = get_balloon_scratch_page();
-#ifdef CONFIG_XEN_HAVE_PVMMU
-		if (xen_pv_domain() && !PageHighMem(page)) {
+		if (!xen_feature(XENFEAT_auto_translated_physmap)) {
+			unsigned long p;
+			struct page   *scratch_page = get_balloon_scratch_page();
+
+			if (!PageHighMem(page)) {
 				ret = HYPERVISOR_update_va_mapping(
 						(unsigned long)__va(pfn << PAGE_SHIFT),
 						pfn_pte(page_to_pfn(scratch_page),
 							PAGE_KERNEL_RO), 0);
 				BUG_ON(ret);
 			}
-#endif
-		if (!xen_feature(XENFEAT_auto_translated_physmap)) {
-			unsigned long p;
 			p = page_to_pfn(scratch_page);
 			__set_phys_to_machine(pfn, pfn_to_mfn(p));
-		}
+
 			put_balloon_scratch_page();
+		}
+#endif
 
 		balloon_append(pfn_to_page(pfn));
 	}
@@ -627,6 +630,7 @@ static int __init balloon_init(void)
 	if (!xen_domain())
 		return -ENODEV;
 
+	if (!xen_feature(XENFEAT_auto_translated_physmap)) {
 		for_each_online_cpu(cpu)
 		{
 			per_cpu(balloon_scratch_page, cpu) = alloc_page(GFP_KERNEL);
@@ -636,6 +640,7 @@ static int __init balloon_init(void)
 			}
 		}
 		register_cpu_notifier(&balloon_cpu_notifier);
+	}
 
 	pr_info("Initialising balloon driver\n");