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 <zhukeqi...@huawei.com> Reviewed-by: Andrew Jones <drjo...@redhat.com> Reviewed-by: Peter Xu <pet...@redhat.com> --- 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. 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); } -- 2.23.0