[Qemu-devel] [PATCH] migration: allow private destination ram with x-ignore-shared

[Peng Tao <tao.peng@linux.alibaba.com>]

By removing the share ram check, qemu is able to migrate
to private destination ram when x-ignore-shared capability
is on. Then we can create multiple destination VMs based
on the same source VM.

This changes the x-ignore-shared migration capability to
work similar to Lai's original bypass-shared-memory
work(https://lists.gnu.org/archive/html/qemu-devel/2018-04/msg00003.html)
which enables kata containers (https://katacontainers.io)
to implement the VM templating feature.

An example usage in kata containers(https://katacontainers.io):
1. Start the source VM:
   qemu-system-x86 -m 2G \
     -object memory-backend-file,id=mem0,size=2G,share=on,mem-path=/tmpfs/template-memory \
     -numa node,memdev=mem0
2. Stop the template VM, set migration x-ignore-shared capability,
   migrate "exec:cat>/tmpfs/state", quit it
3. Start target VM:
   qemu-system-x86 -m 2G \
     -object memory-backend-file,id=mem0,size=2G,share=off,mem-path=/tmpfs/template-memory \
     -numa node,memdev=mem0 \
     -incoming defer
4. connect to target VM qmp, set migration x-ignore-shared capability,
migrate_incoming "exec:cat /tmpfs/state"
5. create more target VMs repeating 3 and 4

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Yury Kotov <yury-kotov@yandex-team.ru>
Cc: Jiangshan Lai <laijs@hyper.sh>
Cc: Xu Wang <xu@hyper.sh>
Signed-off-by: Peng Tao <tao.peng@linux.alibaba.com>
---
 migration/ram.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index 1ca9ba7..cdb82a3 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -3373,7 +3373,6 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
         }
         if (migrate_ignore_shared()) {
             qemu_put_be64(f, block->mr->addr);
-            qemu_put_byte(f, ramblock_is_ignored(block) ? 1 : 0);
         }
     }
 
@@ -4340,12 +4339,6 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
                     }
                     if (migrate_ignore_shared()) {
                         hwaddr addr = qemu_get_be64(f);
-                        bool ignored = qemu_get_byte(f);
-                        if (ignored != ramblock_is_ignored(block)) {
-                            error_report("RAM block %s should %s be migrated",
-                                         id, ignored ? "" : "not");
-                            ret = -EINVAL;
-                        }
                         if (ramblock_is_ignored(block) &&
                             block->mr->addr != addr) {
                             error_report("Mismatched GPAs for block %s "
-- 
1.8.3.1