Re: [Qemu-devel] [PATCH] vmstate: Add VMSTATE_OPAQUE to save/load complex data structures

[Peter Maydell]

On Thu, 23 May 2019 at 18:02, Roman Kiryanov via Qemu-devel
<address@hidden> wrote:
>
> Hi Dave, thank you for looking.
>
> > Can you give me an example of where you would use it?
>
> We use it in our host memory sharing device. I used the existing
> macros for all fields I could, but unfortunately some state does not fit
> into them. We use this new macro to save/load memory
> allocators (for now we have malloc, but we are working on adding
> Vulkan calls). For now the state looks this way:
>
> class Allocator;
> unordered_map<int, Allocator *> state;
>
> class MallocAllocator: public Allocator {
>     unordered_map<int, vector<char>> state;
> };
>
> class VulkanAllocator: public Allocator {
>     // TBD
> };

This is all C++, so it's not relevant for upstream QEMU...

In any case, migration state on the wire needs to be
architecture/endianness/implementation-independent,
so you can't just send raw complex data structures -- you
need to marshal these into something else (by having
pre-load/post-save functions in whatever device is using
these to marshal between the complex data structures and a
plain-old-array-of-whatevers, or by having the migration
code specifically support migration of the complex data
structures.)

thanks
-- PMM