Re: [Qemu-devel] [RFC v2 3/5] vmstate: add VMS_MUST_EXIST

[Michael S. Tsirkin]

On Tue, Mar 25, 2014 at 09:23:13AM +0000, Dr. David Alan Gilbert wrote:
> * Michael S. Tsirkin (address@hidden) wrote:
> > On Mon, Mar 24, 2014 at 05:11:16PM +0000, Dr. David Alan Gilbert wrote:
> 
> <snip>
> 
> > > I think your intent here is just to misuse the field_exist function 
> > > pointer 
> > > as a call for a different reason as a hook for a validator; is it really 
> > > worth
> > > misusing it like that or is something more explicit worth it?
> > > Perhaps something passed an Error** so it could pass back what was wrong?
> > 
> > 
> > Well adding a required field seems valuable by itself, does it not?
> 
> Maybe; however most fields are always-present, unless they have a test
> function or minimum version, so it's a little weird to add a 'required'
> when that's the default.

Right - here we say "there is a test function but it must return true".

I considered adding a separate callback but it worried me
that it's not clear how would it interact with the exist
flag or the version flag. Ideas?

> > And there's no way to pass in Error** since none of the callers
> > has Error**: all of migration still uses stderr to pass
> > errors.
> > 
> > So we could add an API but it doesn't seem too valuable.
> > 
> > Since all callers will use this through a wrapper like VMSTATE_TEST,
> > it will be easy to change our mind later.
> 
> Yep, that's fine - was just an idea.
> 
> Dave
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK