[Qemu-devel] [PATCH RFC v2 3/4] memory: make MemoryRegion alias migratable

[Igor Mammedov <imammedo@redhat.com>]

use qemu_ram_alloc_from_ptr() to create aliased RAMBlock
to the part of original memory region.

Change is migration safe as we do not migrate every existing RAMBlock
anymore, to make it migratable code has to explicitly call
vmstate_register_ram() on MemoryRegion that owns RAMBlock.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
PS:
tested ping-pong migration between new and old QEMU for x86 pc/q35
and s390 machines.

CC: dgilbert@redhat.com

 exec.c   | 9 +++++----
 memory.c | 6 ++++++
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/exec.c b/exec.c
index 3e78de3b8f..f5e9699632 100644
--- a/exec.c
+++ b/exec.c
@@ -2313,7 +2313,7 @@ static void ram_block_add(RAMBlock *new_block, Error **errp, bool shared)
                                         new_block->used_length,
                                         DIRTY_CLIENTS_ALL);
 
-    if (new_block->host) {
+    if (new_block->host && !new_block->mr->alias) {
         qemu_ram_setup_dump(new_block->host, new_block->max_length);
         qemu_madvise(new_block->host, new_block->max_length, QEMU_MADV_HUGEPAGE);
         /* MADV_DONTFORK is also needed by KVM in absence of synchronous MMU */
@@ -2497,7 +2497,7 @@ void qemu_ram_free(RAMBlock *block)
         return;
     }
 
-    if (block->host) {
+    if (block->host && !block->mr->alias) {
         ram_block_notify_remove(block->host, block->max_length);
     }
 
@@ -2671,7 +2671,8 @@ RAMBlock *qemu_ram_block_from_host(void *ptr, bool round_offset,
 
     rcu_read_lock();
     block = atomic_rcu_read(&ram_list.mru_block);
-    if (block && block->host && host - block->host < block->max_length) {
+    if (block && !block->mr->alias && block->host &&
+        host - block->host < block->max_length) {
         goto found;
     }
 
@@ -2680,7 +2681,7 @@ RAMBlock *qemu_ram_block_from_host(void *ptr, bool round_offset,
         if (block->host == NULL) {
             continue;
         }
-        if (host - block->host < block->max_length) {
+        if (!block->mr->alias && host - block->host < block->max_length) {
             goto found;
         }
     }
diff --git a/memory.c b/memory.c
index 5d8c9a9234..d7c3609ce3 100644
--- a/memory.c
+++ b/memory.c
@@ -1678,6 +1678,12 @@ void memory_region_init_alias(MemoryRegion *mr,
     memory_region_init(mr, owner, name, size);
     mr->alias = orig;
     mr->alias_offset = offset;
+    if (orig->ram_block && size) {
+        mr->destructor = memory_region_destructor_ram;
+        mr->ram_block = qemu_ram_alloc_from_ptr(size,
+                                                orig->ram_block->host + offset,
+                                                mr, &error_fatal);
+    }
 }
 
 void memory_region_init_rom_nomigrate(MemoryRegion *mr,
-- 
2.18.1