Re: [Qemu-devel] [very-WIP 1/7] migration: Add VMSTATE_WITH_TMP

[Jianjun Duan <duanj@linux.vnet.ibm.com>]

On 10/17/2016 11:52 AM, Dr. David Alan Gilbert wrote:
> * Jianjun Duan (duanj@linux.vnet.ibm.com) wrote:
>>
>>
>> On 10/16/2016 08:31 PM, David Gibson wrote:
>>> On Tue, Oct 11, 2016 at 06:18:30PM +0100, Dr. David Alan Gilbert (git) wrote:
>>>> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
>>>>
>>>> VMSTATE_WITH_TMP is for handling structures where some calculation
>>>> or rearrangement of the data needs to be performed before the data
>>>> hits the wire.
>>>> For example,  where the value on the wire is an offset from a
>>>> non-migrated base, but the data in the structure is the actual pointer.
>>>>
>>>> To use it, a temporary type is created and a vmsd used on that type.
>>>> The first element of the type must be 'parent' a pointer back to the
>>>> type of the main structure.  VMSTATE_WITH_TMP takes care of allocating
>>>> and freeing the temporary before running the child vmsd.
>>>>
>>>> The post_load/pre_save on the child vmsd can copy things from the parent
>>>> to the temporary using the parent pointer and do any other calculations
>>>> needed; it can then use normal VMSD entries to do the actual data
>>>> storage without having to fiddle around with qemu_get_*/qemu_put_*
>>>>
>> If customized put/get can do transformation and dumping/loading data
>> to/from the parent structure, you don't have to go through
>> pre_save/post_load, and may get rid of parent pointer.
> 
> Yes but I'd rather try and get rid of the customized put/get from
> every device, because then people start using qemu_put/qemu_get in them all.
> 
Then customized handling need to happen in pre_save/post_load. I think
you need a way to pass TMP pointer around?

Thanks,
Jianjun
> Dave
> 
>>
>> Thanks,
>> Jianjun
>>
>>>> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>>>
>>> The requirement for the parent pointer is a little clunky, but I don't
>>> quickly see a better way, and it is compile-time verified.  As noted
>>> elsewhere I think this is a really useful approach which could allow a
>>> bunch of internal state cleanups while preserving migration.
>>>
>>> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
>>>
>>>> ---
>>>>  include/migration/vmstate.h | 20 ++++++++++++++++++++
>>>>  migration/vmstate.c         | 38 ++++++++++++++++++++++++++++++++++++++
>>>>  2 files changed, 58 insertions(+)
>>>>
>>>> diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
>>>> index 9500da1..efb0e90 100644
>>>> --- a/include/migration/vmstate.h
>>>> +++ b/include/migration/vmstate.h
>>>> @@ -259,6 +259,7 @@ extern const VMStateInfo vmstate_info_cpudouble;
>>>>  extern const VMStateInfo vmstate_info_timer;
>>>>  extern const VMStateInfo vmstate_info_buffer;
>>>>  extern const VMStateInfo vmstate_info_unused_buffer;
>>>> +extern const VMStateInfo vmstate_info_tmp;
>>>>  extern const VMStateInfo vmstate_info_bitmap;
>>>>  extern const VMStateInfo vmstate_info_qtailq;
>>>>  
>>>> @@ -651,6 +652,25 @@ extern const VMStateInfo vmstate_info_qtailq;
>>>>      .offset     = offsetof(_state, _field),                          \
>>>>  }
>>>>  
>>>> +/* Allocate a temporary of type 'tmp_type', set tmp->parent to _state
>>>> + * and execute the vmsd on the temporary.  Note that we're working with
>>>> + * the whole of _state here, not a field within it.
>>>> + * We compile time check that:
>>>> + *    That _tmp_type contains a 'parent' member that's a pointer to the
>>>> + *        '_state' type
>>>> + *    That the pointer is right at the start of _tmp_type.
>>>> + */
>>>> +#define VMSTATE_WITH_TMP(_state, _tmp_type, _vmsd) {                 \
>>>> +    .name         = "tmp",                                           \
>>>> +    .size         = sizeof(_tmp_type) +                              \
>>>> +                    QEMU_BUILD_BUG_EXPR(offsetof(_tmp_type, parent) != 0) + \
>>>> +                    type_check_pointer(_state,                       \
>>>> +                        typeof_field(_tmp_type, parent)),            \
>>>> +    .vmsd         = &(_vmsd),                                        \
>>>> +    .info         = &vmstate_info_tmp,                               \
>>>> +    .flags        = VMS_LINKED,                                      \
>>>> +}
>>>> +
>>>>  #define VMSTATE_UNUSED_BUFFER(_test, _version, _size) {              \
>>>>      .name         = "unused",                                        \
>>>>      .field_exists = (_test),                                         \
>>>> diff --git a/migration/vmstate.c b/migration/vmstate.c
>>>> index 2157997..f2563c5 100644
>>>> --- a/migration/vmstate.c
>>>> +++ b/migration/vmstate.c
>>>> @@ -925,6 +925,44 @@ const VMStateInfo vmstate_info_unused_buffer = {
>>>>      .put  = put_unused_buffer,
>>>>  };
>>>>  
>>>> +/* vmstate_info_tmp, see VMSTATE_WITH_TMP, the idea is that we allocate
>>>> + * a temporary buffer and the pre_load/pre_save methods in the child vmsd
>>>> + * copy stuff from the parent into the child and do calculations to fill
>>>> + * in fields that don't really exist in the parent but need to be in the
>>>> + * stream.
>>>> + */
>>>> +static int get_tmp(QEMUFile *f, void *pv, size_t size, VMStateField *field)
>>>> +{
>>>> +    int ret;
>>>> +    const VMStateDescription *vmsd = field->vmsd;
>>>> +    int version_id = field->version_id;
>>>> +    void *tmp = g_malloc(size);
>>>> +
>>>> +    /* Writes the parent field which is at the start of the tmp */
>>>> +    *(void **)tmp = pv;
>>>> +    ret = vmstate_load_state(f, vmsd, tmp, version_id);
>>>> +    g_free(tmp);
>>>> +    return ret;
>>>> +}
>>>> +
>>>> +static void put_tmp(QEMUFile *f, void *pv, size_t size, VMStateField *field,
>>>> +                    QJSON *vmdesc)
>>>> +{
>>>> +    const VMStateDescription *vmsd = field->vmsd;
>>>> +    void *tmp = g_malloc(size);
>>>> +
>>>> +    /* Writes the parent field which is at the start of the tmp */
>>>> +    *(void **)tmp = pv;
>>>> +    vmstate_save_state(f, vmsd, tmp, vmdesc);
>>>> +    g_free(tmp);
>>>> +}
>>>> +
>>>> +const VMStateInfo vmstate_info_tmp = {
>>>> +    .name = "tmp",
>>>> +    .get = get_tmp,
>>>> +    .put = put_tmp,
>>>> +};
>>>> +
>>>>  /* bitmaps (as defined by bitmap.h). Note that size here is the size
>>>>   * of the bitmap in bits. The on-the-wire format of a bitmap is 64
>>>>   * bit words with the bits in big endian order. The in-memory format
>>>
>>
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>