Re: [Qemu-devel] [QEMU RFC PATCH v3 4/6] Migration: migrate QTAILQ

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 07, 2016 at 06:31:41PM +0200, Paolo Bonzini wrote:
> 
> 
> On 07/06/2016 16:43, Dr. David Alan Gilbert wrote:
> >   b) I think you should really try and split it into two parts:
> >     1) A VMSTATE_ARRAY_CUSTOM (?) - so it's an array of elements but we've got a special
> >        way of allocating/counting/reading the elements
> >     2) A version of that which is for a QTAILQ.
> >        It's important that whatever ends up on the migration stream doesn't reflect
> >        that it happens to be implemented as a QTAILQ; so if you decide to change
> >        it later the migration compatibility doesn't break.
> 
> (Just to clarify before Jianjun wakes up) a VMSTATE_ARRAY is just a
> sequence of elements.  The count, if needed as in a VARRAY, is stored
> earlier and separately.

And if you migrate the count, you must validate that
it's not bigger than an actual array size before VARRAY.

>  Currently lists (including this QTAILQ) are
> usually represented in the migration stream as a sequence of elements
> preceded by "1" and terminated by a "0".  Would you like to change it to
> a count + sequence as well?
> 
> This would prevent using the new QTAILQ support for other existing lists
> such as virtio-blk and scsi's request lists.
> 
> >   c) Use new trace_ names for get_qtailq rather than misusing trace_vmstate_load_state*
> >   d) Add a trace_ for put_qtailq as well - that way when it goes horribly wrong we can
> >      turn the tracing on on both sides :-)
> >   e) Is there anyway to make QTAILQ_RAW_INSERT_TAIL any more readable?
> >      I don't think I understand why you can't use the standard QTAILQ macros.
> 
> Because they assume a particular type. The "raw" version need to work on
> void*.
> 
> Thanks,
> 
> Paolo
> 
> >   f) You might need to fix up Amit's scripts/vmstate-static-checker.py