[Qemu-devel] [RFC/PATCH] migration: SMRAM dirty bitmap not fetched from kvm-kmod and not send to destination

[Herongguang (Stephen)]

Hi,
We found a problem that when a redhat 6 VM reboots (in grub countdown UI), 
migrating this VM will result in VM’s memory difference between source and 
destination side. The difference always resides in GPA 0xA0000~0xC0000, i.e. 
SMRAM area.

Occasionally this result in VM instruction emulation error in destination side.

After some digging, I think this is because in migration code, in 
migration_bitmap_sync(), only memory slots in address space 
address_space_memory’s dirty bitmap  fetched from kvm-kmod, while SMRAM memory 
slot, in address space smram_address_space’s dirty bitmap not fetched from 
kvm-kmod, thus modifications in SMRAM in source side are not sent to 
destination side.

I tried following patch, and this phenomenon does not happen anymore. Do you 
think this patch is OK or do you have better idea? Thanks.

diff --git a/migration/ram.c b/migration/ram.c
index a3d70c4..1cc4360 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -607,6 +607,8 @@ static void migration_bitmap_sync_init(void)
     iterations_prev = 0;
 }

+extern AddressSpace smram_address_space;
+
 static void migration_bitmap_sync(void)
 {
     RAMBlock *block;
@@ -627,6 +629,7 @@ static void migration_bitmap_sync(void)

     trace_migration_bitmap_sync_start();
     address_space_sync_dirty_bitmap(&address_space_memory);
+    address_space_sync_dirty_bitmap(&smram_address_space);

     qemu_mutex_lock(&migration_bitmap_mutex);
     rcu_read_lock();
diff --git a/target-i386/kvm.c b/target-i386/kvm.c
index d1a25c5..b98fe22 100644
--- a/target-i386/kvm.c
+++ b/target-i386/kvm.c
@@ -1111,7 +1111,7 @@ static int kvm_get_supported_msrs(KVMState *s)

 static Notifier smram_machine_done;
 static KVMMemoryListener smram_listener;
-static AddressSpace smram_address_space;
+AddressSpace smram_address_space;
 static MemoryRegion smram_as_root;
 static MemoryRegion smram_as_mem;