Re: [PATCH v3] migration: Support gtree migration

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Auger Eric (eric.auger@redhat.com) 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)


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