Peter Maydell <[email protected]> wrote:
> On 7 August 2018 at 15:32, Juan Quintela <[email protected]> wrote:
>> Peter Maydell <[email protected]> wrote:
>>> On 7 August 2018 at 14:17, Juan Quintela <[email protected]> wrote:
>>> Also, what prompted me to write this patch was that for 3.0
>>> I had to fix several bugs where subsections had been written
>>> with no needed function and the migration code was (a)
>>> silently ignoring the subsection and (b) silently ignoring
>>> any subsection following. If you want to make it a bug to
>>
>> b) is clearly a bug.
>>
>> a) ... I am not sure that this should be correct.
>
> I can see these plausible API choices:
> (1) .needed for a subsection VMSD has the same behaviour as
> .needed for a top-level VMSD: if not present, means "always"
> (2) .needed is mandatory for a subsection VMSD, and failure
> to provide it is a bug which we diagnose as early as
> possible
I vote for (2)
> If I understand you correctly you're suggesting
> (3) .needed is not mandatory and if not present means
> "never transfer the subsection"
> which I think would be confusing.
No. I am suggestion that not having "needed" is a bug.
> I could live with (2) but I would end up writing a lot
> of .needed functions like
>
> static bool needed_always(void *opaque)
> {
> return true;
> }
vmstate_needed_always() can be exported.
>
> which seems a bit unnecessary. (I have another patchset
> in the works which will have one of these, assuming this
> patch doesn't go into master first.)
But what I wonder is _why_ we want subsections that are always there.
We have:
- normal sections
int32_t foo
int64_t bad;
- subsections (always optional)
if needed(whatever)
int16_t foo2
int32_t bar2
So needed_always means that it is never optional, it is always sent.
Here we have two cases:
- migration from old qemu (subsction don't exist) to new qemu works
then the subsection clearly is mandatory
- migration from old qemu to new qemu don't work, because there are
missing bits:
then there is not problem adding the fields to the toplevel section
and increasing the version number, because we can't migrate from
older versions.
I can't see how to can make both claims true at the same time.
Later, Juan.
