Re: [PATCH v3] migration: Support gtree migration

[Auger Eric]

Hi,

On 10/10/19 8:53 PM, Dr. David Alan Gilbert wrote:
> * Auger Eric (address@hidden) wrote:
>> Hi peter,
>>
>> On 10/10/19 2:35 PM, Peter Xu wrote:
>>> On Thu, Oct 10, 2019 at 02:11:46PM +0200, Auger Eric wrote:
>>>>>>> Also, should we avoid using UINT in all cases?  But of course if we
>>>>>>> start to use VMSTATE_UINT32_V then we don't have this issue.
>>>>>> Depending on the clarification of above point, maybe I can rename
>>>>>> VMSTATE_GTREE_DIRECT_KEY_V into VMSTATE_GTREE_DIRECT_UINT_KEY_V
>>>>>>
>>>>>> direct keys seem to be more common for hash tables actually.
>>>>>> https://developer.gnome.org/glib/stable/glib-Hash-Tables.html#g-hash-table-new-full
>>>>>>
>>>>>> There are stand conversion macros to/from int, uint, size
>>>>>> https://developer.gnome.org/glib/stable/glib-Type-Conversion-Macros.html
>>>>>
>>>>> Yeh it's fine to use direct keys.  Though my question was more about
>>>>> "unsigned int" - here when we put, we cast a pointer into unsigned
>>>>> int, which can be 2/4 bytes, IIUC.  I'm thinking whether at least we
>>>>> should use direct cast (e.g., (uint32_t)ptr) to replace
>>>>> GPOINTER_TO_UINT() to make sure it's 4 bytes.  Futher, maybe we should
>>>>> start with uint64_t here in the migration stream, otherwise we should
>>>>> probably drop the high 32 bits if we migrate a gtree whose key is 64
>>>>> bits long (and since we're working with migration we won't be able to
>>>>> change that in the future for compatibility reasons...).
>>>>>
>>>>> Summary:
>>>>>
>>>>> Maybe we can replace:
>>>>>
>>>>>     qemu_put_be32(f, GPOINTER_TO_UINT(key)); /* direct key */
>>>>>
>>>>> To:
>>>>>
>>>>>     qemu_put_be64(f, (uint64_t)key); /* direct key */
>>>>>
>>>>> And apply similar thing to get() side?
>>>>
>>>> This was my first idea as well but I got stuck with a mingw compilation
>>>> issues if I remember correctly, trying to cast pointers to a wrong sized
>>>> uint. This got removed by using the GPOINTER_TO_UINT conversion functions.
>>>
>>> #define GPOINTER_TO_UINT(p) ((guint) (gulong) (p))
>>>
>>> Could "(uint64_t)(uintptr_t)pointer" do the work?
>> the problem rather is on the get side, when you cast the uint64_t value
>> into the pointer. it does not compile with
>>
>> make docker-test-mingw@fedora
>>
>>
>> /tmp/qemu-test/src/migration/vmstate-types.c:800:19: error: cast to
>> pointer from integer of different size [-Werror=int-to-pointer-cast]
>>              key = (void *)(uint64_t)qemu_get_be64(f);
>>
>> I would be tempted to rename the macro to emphasize the key is an
>> uint32. It should cover most of the use cases, no?
> 
> Try:
>     (void *)(uintptr_t)qemu_get_be64(f)
This indeed compiles with mingw. Thanks to both of you for the hint. I
was not familiar with uintptr_t.

Thanks

Eric
> 
> 
>>
>> Thanks
>>
>> Eric
>>
>>
>>>
>>> Thanks,
>>>
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK
>