[v2,1/2] accel: kvm: Fix memory waste under mismatch page size

Commit Message

zhukeqian Dec. 17, 2020, 1:49 a.m. UTC

When handle dirty log, we face qemu_real_host_page_size and
TARGET_PAGE_SIZE. The first one is the granule of KVM dirty
bitmap, and the second one is the granule of QEMU dirty bitmap.

As qemu_real_host_page_size >= TARGET_PAGE_SIZE (kvm_init()
enforced it), misuse TARGET_PAGE_SIZE to init kvmslot dirty_bmap
may waste memory. For example, when qemu_real_host_page_size is
64K and TARGET_PAGE_SIZE is 4K, it wastes 93.75% (15/16) memory.

Signed-off-by: Keqian Zhu <[e-mail]>
Reviewed-by: Andrew Jones <[e-mail]>
Reviewed-by: Peter Xu <[e-mail]>
---
 accel/kvm/kvm-all.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

---

v2
 - Address Andrew's comment (qemu_real_host_page_size >= TARGET_PAGE_SIZE
   is a rule).
 - Add Andrew and Peter's R-b.

Patch

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 389eaace72..f6b16a8df8 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -620,8 +620,12 @@  static void kvm_memslot_init_dirty_bitmap(KVMSlot *mem)
      * too, in most cases).
      * So for now, let's align to 64 instead of HOST_LONG_BITS here, in
      * a hope that sizeof(long) won't become >8 any time soon.
+     *
+     * Note: the granule of kvm dirty log is qemu_real_host_page_size.
+     * And mem->memory_size is aligned to it (otherwise this mem can't
+     * be registered to KVM).
      */
-    hwaddr bitmap_size = ALIGN(((mem->memory_size) >> TARGET_PAGE_BITS),
+    hwaddr bitmap_size = ALIGN(mem->memory_size / qemu_real_host_page_size,
                                         /*HOST_LONG_BITS*/ 64) / 8;
     mem->dirty_bmap = g_malloc0(bitmap_size);
 }