[Qemu-devel] [PATCH] Avoid loading ram pages that are all zeroes

[Avi Kivity <avi@redhat.com>]

Restoring or migrating zero pages causes unnecessary allocation of memory.
We can avoid this by dropping the loaded page instead of memset()ing it.

Signed-off-by: Avi Kivity <avi@redhat.com>

diff --git a/vl.c b/vl.c
index 7b58605..42e97c9 100644
--- a/vl.c
+++ b/vl.c
@@ -3019,6 +3019,19 @@ void qemu_del_wait_object(HANDLE handle, WaitObjectFunc *func, void *opaque)
 /***********************************************************/
 /* ram save/restore */
 
+static void drop_page(uint8_t *page)
+{
+    if (TARGET_PAGE_SIZE < getpagesize())
+        return;
+#ifdef CONFIG_KVM
+    if (kvm_enabled() && !kvm_has_sync_mmu())
+        return;
+#endif
+#ifdef MADV_DONTNEED
+    madvise(page, TARGET_PAGE_SIZE, MADV_DONTNEED);
+#endif
+}
+
 static int ram_get_page(QEMUFile *f, uint8_t *buf, int len)
 {
     int v;
@@ -3032,6 +3045,8 @@ static int ram_get_page(QEMUFile *f, uint8_t *buf, int len)
     case 1:
         v = qemu_get_byte(f);
         memset(buf, v, len);
+        if (v == 0)
+            drop_page(buf);
         break;
     default:
         return -EINVAL;
@@ -3289,6 +3304,8 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
         if (flags & RAM_SAVE_FLAG_COMPRESS) {
             uint8_t ch = qemu_get_byte(f);
             memset(phys_ram_base + addr, ch, TARGET_PAGE_SIZE);
+            if (ch == 0)
+                drop_page(phys_ram_base + addr);
         } else if (flags & RAM_SAVE_FLAG_PAGE)
             qemu_get_buffer(f, phys_ram_base + addr, TARGET_PAGE_SIZE);
     } while (!(flags & RAM_SAVE_FLAG_EOS));