memory: Skip dirty tracking for un-migratable memory regions

It makes no sense to track dirty pages for those un-migratable memory
regions (e.g., Memory BAR region of the VFIO PCI device) and doing so
will potentially lead to some unpleasant issues during migration [1].

Skip dirty tracking for those regions by evaluating if the region is
migratable before setting dirty_log_mask (DIRTY_MEMORY_MIGRATION).

[1] https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg03757.html

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Message-Id: <20201116132210.1730-1-yuzenghui@huawei.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

softmmu/memory.c

@@ -1806,7 +1806,10 @@ bool memory_region_is_ram_device(MemoryRegion *mr)
 uint8_t memory_region_get_dirty_log_mask(MemoryRegion *mr)
 {
     uint8_t mask = mr->dirty_log_mask;
-    if (global_dirty_log && (mr->ram_block || memory_region_is_iommu(mr))) {
+    RAMBlock *rb = mr->ram_block;
+
+    if (global_dirty_log && ((rb && qemu_ram_is_migratable(rb)) ||
+                             memory_region_is_iommu(mr))) {
         mask |= (1 << DIRTY_MEMORY_MIGRATION);
     }
     return mask;