migration/ram: Simplify host page handling in ram_load_postcopy()

Add two new helper functions. This will come in come handy once we want to
handle ram block resizes while postcopy is active.

Note that ram_block_from_stream() will already print proper errors.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210429112708.12291-8-david@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  dgilbert: Added brackets in host_page_from_ram_block_offset
     to cause uintptr_t to cast the sum, to fix armhf-cross build

migration/ram.c

@@ -3085,6 +3085,20 @@ static inline void *host_from_ram_block_offset(RAMBlock *block,
     return block->host + offset;
 }
 
+static void *host_page_from_ram_block_offset(RAMBlock *block,
+                                             ram_addr_t offset)
+{
+    /* Note: Explicitly no check against offset_in_ramblock(). */
+    return (void *)QEMU_ALIGN_DOWN((uintptr_t)(block->host + offset),
+                                   block->page_size);
+}
+
+static ram_addr_t host_page_offset_from_ram_block_offset(RAMBlock *block,
+                                                         ram_addr_t offset)
+{
+    return ((uintptr_t)block->host + offset) & (block->page_size - 1);
+}
+
 static inline void *colo_cache_from_block_offset(RAMBlock *block,
                              ram_addr_t offset, bool record_bitmap)
 {
@@ -3481,13 +3495,12 @@ static int ram_load_postcopy(QEMUFile *f)
     MigrationIncomingState *mis = migration_incoming_get_current();
     /* Temporary page that is later 'placed' */
     void *postcopy_host_page = mis->postcopy_tmp_page;
-    void *this_host = NULL;
+    void *host_page = NULL;
     bool all_zero = true;
     int target_pages = 0;
 
     while (!ret && !(flags & RAM_SAVE_FLAG_EOS)) {
         ram_addr_t addr;
-        void *host = NULL;
         void *page_buffer = NULL;
         void *place_source = NULL;
         RAMBlock *block = NULL;
@@ -3512,9 +3525,12 @@ static int ram_load_postcopy(QEMUFile *f)
         if (flags & (RAM_SAVE_FLAG_ZERO | RAM_SAVE_FLAG_PAGE |
                      RAM_SAVE_FLAG_COMPRESS_PAGE)) {
             block = ram_block_from_stream(f, flags);
+            if (!block) {
+                ret = -EINVAL;
+                break;
+            }
 
-            host = host_from_ram_block_offset(block, addr);
-            if (!host) {
+            if (!offset_in_ramblock(block, addr)) {
                 error_report("Illegal RAM offset " RAM_ADDR_FMT, addr);
                 ret = -EINVAL;
                 break;
@@ -3532,19 +3548,17 @@ static int ram_load_postcopy(QEMUFile *f)
              * of a host page in one chunk.
              */
             page_buffer = postcopy_host_page +
-                          ((uintptr_t)host & (block->page_size - 1));
+                          host_page_offset_from_ram_block_offset(block, addr);
+            /* If all TP are zero then we can optimise the place */
             if (target_pages == 1) {
-                this_host = (void *)QEMU_ALIGN_DOWN((uintptr_t)host,
-                                                    block->page_size);
-            } else {
+                host_page = host_page_from_ram_block_offset(block, addr);
+            } else if (host_page != host_page_from_ram_block_offset(block,
+                                                                    addr)) {
                 /* not the 1st TP within the HP */
-                if (QEMU_ALIGN_DOWN((uintptr_t)host, block->page_size) !=
-                    (uintptr_t)this_host) {
-                    error_report("Non-same host page %p/%p",
-                                  host, this_host);
-                    ret = -EINVAL;
-                    break;
-                }
+                error_report("Non-same host page %p/%p", host_page,
+                             host_page_from_ram_block_offset(block, addr));
+                ret = -EINVAL;
+                break;
             }
 
             /*
@@ -3623,16 +3637,11 @@ static int ram_load_postcopy(QEMUFile *f)
         }
 
         if (!ret && place_needed) {
-            /* This gets called at the last target page in the host page */
-            void *place_dest = (void *)QEMU_ALIGN_DOWN((uintptr_t)host,
-                                                       block->page_size);
-
             if (all_zero) {
-                ret = postcopy_place_page_zero(mis, place_dest,
-                                               block);
+                ret = postcopy_place_page_zero(mis, host_page, block);
             } else {
-                ret = postcopy_place_page(mis, place_dest,
-                                          place_source, block);
+                ret = postcopy_place_page(mis, host_page, place_source,
+                                          block);
             }
             place_needed = false;
             target_pages = 0;