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

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

On 10/17/2016 12:16 PM, Dr. David Alan Gilbert wrote:
> * Jianjun Duan (duanj@linux.vnet.ibm.com) wrote:
>>
>>
>> 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?
> 
> But then why is that better than having the parent pointer?
> 
IIUC, from the put_tmp, I didn't see how tmp is filled with data. I
suppose it is to be filled by pre_save. So tmp pointer needs to find a
way from inside pre_save to put_tmp. How does it happen?

Thanks,
Jianjun


> Dave
> 
>>
>> 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
>>>
>>
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>