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Re: [Qemu-devel] [PATCH] migration: Correctly handle subsections with no

From: Juan Quintela
Subject: Re: [Qemu-devel] [PATCH] migration: Correctly handle subsections with no 'needed' function
Date: Tue, 07 Aug 2018 16:49:10 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)

Peter Maydell <address@hidden> wrote:
> On 7 August 2018 at 15:32, Juan Quintela <address@hidden> wrote:
>> Peter Maydell <address@hidden> wrote:
>>> On 7 August 2018 at 14:17, Juan Quintela <address@hidden> 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.

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