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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peng Tao (tao.peng@linux.alibaba.com) wrote:
> 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>

I think this is OK, but please post this to qemu-devel.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.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
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK