* Paolo Bonzini (pbonz...@redhat.com) wrote: > > > On 06/10/2016 13:56, Dr. David Alan Gilbert wrote: > >> > Yes, it's sickening but that's what you do to honor backwards > >> > compatibility. > > Actually, that's not *that* bad an idea. > > > > Lets go with Jianjun's structure for the moment; we can always expand on it. > > > > It seems we have ~3 concepts that feel partially independent: > > > > a) The format of the loop on the wire (eg one byte per iteration, 0 > > terminates) > > b) The way the list is represented (QTAILQ, simple array, device > > specific linked-list) > > c) The data gathered in each iteration > > d) The allocation of (c) > > > > This patch has a,b,d all wrapped up together in the get/put functions - > > where I was hoping to find a way to separate them a bit so that we > > could say; I want a loop, with this format, into this data structure, using > > this allocator. > > Yes, the sickening part is when the format of the loop intersects with > the format of the datastructure.
Yes. > I agree with moving the allocator out of VMStateInfo and back into > VMStateField, but only as long as VMStateAllocator could replace other > VMS_* flags. > > I'm not sure about the value in separating (a) and (b), but we can do > things one step at a time. The other observation is that in many of the cases the loop body uses some state present in the outer state or in a value read prior to the start of the loop. For example virtio_blk_load_device uses the vdev pointer inside the loop during the initialisation of each loaded request (I can see some hacky ways of avoiding it but it's messy). > By the way, regarding this: > > > The other possibility is just to bump the version and make the SCSI > > request flag a separate byte after the "is there another entry" byte. > > There is another way to do it that is much more backwards-compatible. > Choose a "default" value of retry corresponding to what QEMU encodes as > a "1". If it's different, use a subsection to encode that. Migration > from old to new will fail if the wrong value of retry is used, because > it will see a 2 where the QTAILQ loop expects a zero or one. Migration > from new to old will fail if the wrong value of retry is used, because > it will see a subsection header where the QTAILQ loop expects a zero or one. > > I think this is acceptable, and it would only affect migration of USB > storage devices. Yes, it might be worth it in some of these cases; although I do try and avoid breaking format at almost all costs. Dave > Paolo -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
