Re: [PATCH for-5.0] vl.c: fix migration failure for 3.1 and older machine types

[Lukáš Doktor]

Dne 04. 03. 20 v 18:27 Igor Mammedov napsal(a):
> Migration from QEMU(v4.0) fails when using 3.1 or older machine
> type. For example if one attempts to migrate
> QEMU-2.12 started as
>   qemu-system-ppc64 -nodefaults -M pseries-2.12 -m 4096 -mem-path /tmp/
> to current master, it will fail with
>   qemu-system-ppc64: Unknown ramblock "ppc_spapr.ram", cannot accept migration
>   qemu-system-ppc64: error while loading state for instance 0x0 of device 
> 'ram'
>   qemu-system-ppc64: load of migration failed: Invalid argument
> 
> Caused by 900c0ba373 commit which switches main RAM allocation to
> memory backends and the fact in 3.1 and older QEMU, backends used
> full[***] QOM path as memory region name instead of backend's name.
> That was changed after 3.1 to use prefix-less names by default
> (fa0cb34d22) for new machine types.
> *** effectively makes main RAM memory region names defined by
> MachineClass::default_ram_id being altered with '/objects/' prefix
> and therefore migration fails as old QEMU sends prefix-less
> name while new QEMU expects name with prefix when using 3.1 and
> older machine types.
> 
> Fix it by forcing implicit[1] memory backend to always use
> prefix-less names for its memory region by setting
>   'x-use-canonical-path-for-ramblock-id'
> property to false.
> 
> 1) i.e. memory backend created by compat glue which maps
> -m/-mem-path/-mem-prealloc/default RAM size into
> appropriate backend type/options to match old CLI format.
> 
> Fixes: 900c0ba373
> Signed-off-by: Igor Mammedov <address@hidden>
> Reported-by: Lukáš Doktor <address@hidden>
> ---
> CC: address@hidden
> CC: address@hidden
> CC: address@hidden
> ---
>  softmmu/vl.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/softmmu/vl.c b/softmmu/vl.c
> index 5549f4b619..1101b1cb41 100644
> --- a/softmmu/vl.c
> +++ b/softmmu/vl.c
> @@ -2800,6 +2800,9 @@ static void create_default_memdev(MachineState *ms, 
> const char *path)
>      object_property_set_int(obj, ms->ram_size, "size", &error_fatal);
>      object_property_add_child(object_get_objects_root(), mc->default_ram_id,
>                                obj, &error_fatal);
> +    /* Ensure backend's memory region name is equal to mc->default_ram_id */
> +    object_property_set_bool(obj, false, 
> "x-use-canonical-path-for-ramblock-id",
> +                             &error_fatal);
>      user_creatable_complete(USER_CREATABLE(obj), &error_fatal);
>      object_unref(obj);
>      object_property_set_str(OBJECT(ms), mc->default_ram_id, "memory-backend",
> 

Tested-by: Lukáš Doktor <address@hidden> 

I can't say anything about the code, but functional-wise it addresses the issue 
and fixes post_copy hugepages migration in my mini-CI (variants 2.10, 2.12, 
3.0, 3.1, 4.0 -> master and vice-versa are failing with master and passing with 
this patch).

Regards,
Lukáš

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