Re: [Qemu-devel] [RFC PATCH 2/3] vmstate: error hint for failed equal checks part 2

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Halil Pasic (pasic@linux.vnet.ibm.com) wrote:
> 
> 
> On 06/07/2017 07:10 PM, Dr. David Alan Gilbert wrote:
> > * Halil Pasic (pasic@linux.vnet.ibm.com) wrote:
> >>
> >>
> >> On 06/07/2017 02:01 PM, Dr. David Alan Gilbert wrote:
> >>> * Halil Pasic (pasic@linux.vnet.ibm.com) wrote:
> >>>>
> >>>>
> >>>> On 06/07/2017 01:07 PM, Dr. David Alan Gilbert wrote:
> >>>>> * Halil Pasic (pasic@linux.vnet.ibm.com) wrote:
> >>>>>> Verbose error reporting for the _EQUAL family. Modify the standard _EQUAL
> >>>>>> so the hint states the assertion probably failed due to a bug. Introduce
> >>>>>> _EQUAL_HINT for specifying a context specific hint.
> >>>>>>
> >>>>>> Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
> >>>>>
> >>>>> I'd prefer not to print 'Bug!?' by default - they already get the
> >>>>> message telling them something didn't match and the migration fails.
> >>>>> There are none-bug ways of this happening, e.g. a user starting a VM on
> >>>>> the source and destination with different configs.
> >>>>
> >>>> I admit, my objective with 'Bug!?' was to provoke. My train of thought is
> >>>> to encourage the programmer to think about and document the circumstances
> >>>> under which such an assertion is supposed to fail (and against which it
> >>>> is supposed to guard).
> >>>>
> >>>> I do not know how skillful are our users but a 4 != 5 then maybe a name
> >>>> of a vmstate field is probably quite scary and not very revealing. I doubt
> >>>> a non qemu developer can use it for something else that reporting a bug.
> >>>>
> >>>> Consequently if there are non-bug ways one can use the hint and state them.
> >>>> Your example with the misconfigured target, by the way, is IMHO also be due
> >>>> to a bug of the management software IMHO.
> >>>>
> >>>> To sum it up: IMHO the message provided by a failing _EQUAL is to ugly
> >>>> and Qemuspeak to be presented to an user-user in non-bug cases. Agree?
> >>>> Disagree?
> >>>
> >>> Disagree.
> >>>
> >>> I don't mind giving field names etc; they make it easy for us as
> >>> developers to track down what's happening, but also sometimes they help
> >>> endusers work around a prolem or see where the problem is; of course
> >>> that varies depending on the field name, but some of our names are
> >>> reasonable (e.g. there's a VMSTATE_INT32_EQUAL on 'queue_size' in
> >>> vmmouse.c).  They're also pretty good if two end users hit the same
> >>> problem they can see the same error message in a bug report.
> >>>
> >>> We often have customer-facing support people look at logs before they
> >>> get as far as us developers; if we have bugs that are 
> >>> 'if it's a failing BLAH device complaining about the BAR field'
> >>> then this fixes it, then that helps them find workarounds/fixes quickly
> >>> even if they don't understand what the BAR field is.
> >>>
> >>
> >> You seem to forget, that I'm not proposing omitting this information,
> >> but extending it with something civilized so one can distinguish between
> >> an assert failed should have never happened situation an a as good as
> >> reasonable error handling for an expected error scenario. IMHO the current
> >> EQUAL looks more like the former (assert) and less like the later (error
> >> reporting for an expected error scenario). Agree? Dissagree?
> > 
> > Yes, the current EQUAL is very terse; but we can't actually tell from
> > the use which case it is; it'll all work nicely when people actually add
> > the correct hint text in useful locations.
> > 
> 
> You are right.
> 
> Since Juan also requested the adding an extra param to the original
> macros variant I will go with that.
> 
> I shied away form it in the first place because I did not want to
> bother the users of the macros without clarifying with the migration
> gurus how the new interface should look like.

If you just make the existing callers pass NULL then that's fine.

Dave

> Thanks a lot!
> 
> Regards,
> Halil
> 
> >> Having a field name is great! That's beyond discussion.
> >>
> >> I see, my 'sum it up' above was a bit unfortunate: it sounds like I'm
> >> against the inclusion of technical info and not against a lack of non
> >> technical info. Sorry for that!
> > 
> > No, that's fine.
> > 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK