Re: [Qemu-devel] [PATCH v3 09/12] kvm: Persistent per kvmslot dirty bitmap

[Peter Xu <peterx@redhat.com>]

On Thu, May 30, 2019 at 02:53:30PM +0100, Dr. David Alan Gilbert wrote:
> * Peter Xu (peterx@redhat.com) wrote:
> > When synchronizing dirty bitmap from kernel KVM we do it in a
> > per-kvmslot fashion and we allocate the userspace bitmap for each of
> > the ioctl.  This patch instead make the bitmap cache be persistent
> > then we don't need to g_malloc0() every time.
> > 
> > More importantly, the cached per-kvmslot dirty bitmap will be further
> > used when we want to add support for the KVM_CLEAR_DIRTY_LOG and this
> > cached bitmap will be used to guarantee we won't clear any unknown
> > dirty bits otherwise that can be a severe data loss issue for
> > migration code.
> > 
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> 
> Is there no way to make this get allocated the first time it's needed?
> I'm thinking here of the VM most of the time not being migrated so we're
> allocating this structure for no benefit.

Valid argument...  sure we can do the allocation on first usage. How
about below change squashed?

(I'll squash them properly into different patches where proper; the
 assertion would belong to the other patch)

=======================

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 23895a95a2..80bc4be03a 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -523,6 +523,11 @@ static int kvm_physical_sync_dirty_bitmap(KVMMemoryListener *kml,                                                                       
             goto out;
         }

+        if (!mem->dirty_bmap) {
+            /* Allocate on the first log_sync, once and for all */
+            mem->dirty_bmap = g_malloc0(bmap_size);
+        }
+
         d.dirty_bitmap = mem->dirty_bmap;
         d.slot = mem->slot | (kml->as_id << 16);
         if (kvm_vm_ioctl(s, KVM_GET_DIRTY_LOG, &d) == -1) {
@@ -638,6 +643,8 @@ static int kvm_physical_log_clear(KVMMemoryListener *kml,
      */

     assert(bmap_start % BITS_PER_LONG == 0);
+    /* We should never do log_clear before log_sync */
+    assert(mem->dirty_bmap);
     if (start_delta) {
         /* Slow path - we need to manipulate a temp bitmap */
         bmap_clear = bitmap_new(bmap_npages);
@@ -995,7 +1002,6 @@ static void kvm_set_phys_mem(KVMMemoryListener *kml,
     mem->start_addr = start_addr;
     mem->ram = ram;
     mem->flags = kvm_mem_flags(mr);
-    mem->dirty_bmap = g_malloc0(bmap_size);

     err = kvm_set_user_memory_region(kml, mem, true);
     if (err) {

Regards,

-- 
Peter Xu