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

[Peter Maydell]

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

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.

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;
}

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.)

thanks
-- PMM