Re: [Qemu-devel] QMP: RFC: I/O error info & query-stop-reason

[Luiz Capitulino <lcapitulino@redhat.com>]

On Thu, 02 Jun 2011 09:02:30 -0500
Anthony Liguori <anthony@codemonkey.ws> wrote:

> On 06/02/2011 08:24 AM, Jiri Denemark wrote:
> > On Thu, Jun 02, 2011 at 08:08:35 -0500, Anthony Liguori wrote:
> >> On 06/02/2011 04:06 AM, Daniel P. Berrange wrote:
> >>>>> B. query-stop-reason
> >>>>> --------------------
> >>>>>
> >>>>> I also have a simple solution for item 2. The vm_stop() accepts a reason
> >>>>> argument, so we could store it somewhere and return it as a string, like:
> >>>>>
> >>>>> ->    { "execute": "query-stop-reason" }
> >>>>> <- { "return": { "reason": "user" } }
> >>>>>
> >>>>> Valid reasons could be: "user", "debug", "shutdown", "diskfull" (hey,
> >>>>> this should be "ioerror", no?), "watchdog", "panic", "savevm", "loadvm",
> >>>>> "migrate".
> >>>>>
> >>>>> Also note that we have a STOP event. It should be extended with the
> >>>>> stop reason too, for completeness.
> >>>>
> >>>>
> >>>> Can we just extend query-block?
> >>>
> >>> Primarily we want 'query-stop-reason' to tell us what caused the VM
> >>> CPUs to stop. If that reason was 'ioerror', then 'query-block' could
> >>> be used to find out which particular block device(s) caused the IO
> >>> error to occurr&   get the "reason" that was in the BLOCK_IO_ERROR
> >>> event.
> >>
> >> My concern is that we're over abstracting here.  We're not going to add
> >> additional stop reasons in the future.
> >>
> >> Maybe just add an 'io-error': True to query-state.
> >
> > Sure, adding a new field to query-state response would work as well. And it
> > seems like a good idea to me since one already needs to call query-status to
> > check if CPUs are stopped or not so it makes sense to incorporate the
> > additional information there as well. And if you want to be safe for the
> > future, the new field doesn't have to be boolean 'io-error' but it can be the
> > string 'reason' which Luiz suggested above.
> 
> 
> String enumerations are a Bad Thing.  It's impossible to figure out what 
> strings are valid and it lacks type safety.
> 
> Adding more booleans provides better type safety, and when we move to 
> QAPI with a queryable schema, provides a way to figure out exactly what 
> combinations are supported by QEMU.

To summarize:

 1. Add a 'io-error' field to query-status (which is only present if
    field 'running' is false)

 2. Extend query-block to contain error information associated with the
    device. This is interesting, because this information will be available
    even if the error didn't cause the VM to stop

Seems good enough to me, comments?