Re: [PATCH v3] migration: Support gtree migration

[Auger Eric <eric.auger@redhat.com>]

Hi Peter,

On 10/10/19 1:35 PM, Peter Xu wrote:
> On Thu, Oct 10, 2019 at 09:57:01AM +0200, Auger Eric wrote:
>>>> +static gboolean put_gtree_elem(gpointer key, gpointer value, gpointer data)
>>>> +{
>>>> +    struct put_gtree_data *capsule = (struct put_gtree_data *)data;
>>>> +    QEMUFile *f = capsule->f;
>>>> +    int ret;
>>>> +
>>>> +    qemu_put_byte(f, true);
>>>> +
>>>> +    /* put the key */
>>>> +    if (!capsule->key_vmsd) {
>>>> +        qemu_put_be32(f, GPOINTER_TO_UINT(key)); /* direct key */
>>>
>>> This is special code path for direct key case.  Can we simply define
>>> VMSTATE_GTREE_DIRECT_KEY_V() somehow better so that it just uses the
>>> VMSTATE_UINT32_V() as the key vmsd?  Then iiuc vmstate_save_state()
>>> could work well with that too.
>> if the key_vmsd is a VMSTATE_UINT32_V then I understand
>> vmstate_save_state(f, capsule->key_vmsd, key, capsule->vmdesc)
>> expects key to be a pointer to a uint32. But in that case of direct key,
>> it is a uint32. I don't figure out how to use vmstate_save_state in your
>> proposal.
> 
> Ah I see the point.  And indeed I can't think of a better way now
> (e.g., maybe I will always try to use GTrees with malloc()ed keys to
> be simple when I want to migrate a gtree, but yeah that's not a reason
> to refuse this patch :).
> 
> Though we should be very careful on defining vmsds for GTrees in the
> future with the help of this patch, and we must have the type (either
> direct or not) to match the real usage of the tree otherwise QEMU can
> potentially unreference the constant as a pointer.
> 
>>
>>>
>>> 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.

Thanks

Eric
> 
> Thanks,
>