Re: [Qemu-devel] [PATCH v3 5/7] hmp: Add info commands for preconfig

[Daniel P . Berrangé]

On Wed, Jun 13, 2018 at 10:47:45AM -0300, Eduardo Habkost wrote:
> On Tue, Jun 12, 2018 at 09:49:14AM +0100, Dr. David Alan Gilbert wrote:
> [...]
> > > > People have been trying to add qom-get etc for quite a while (I tried a
> > > > couple of years ago); it gets stuck in type display issues.  I've not
> > > > directly seen a need for those other variants, but qom-get is something
> > > > I'd love to have, still that's a job for another patch.
> > > 
> > > Yes.
> > > 
> > > > 'info qom-tree' is very very useful when debugging qemu to see what the
> > > > basic state we're building is; it's primarily for debugging.
> > > 
> > > I'm not at all opposed to enabling qom-tree, but I want its QMP building
> > > blocks enabled as well then.  I think enabling their HMP buddies as well
> > > would only make sense.
> > 
> > Hmm; OK, enabling qom-list, qom-get, qom-set, qom-list-types,
> > qom-list-properties for qmp.
> > qom-list and qom-set enabled in HMP.
> 
> I would prefer to avoid enabling qom-set in preconfig mode unless
> we have a good reason for that.  I don't trust all existing
> property setters to not crash or break if the machine is not
> initialized yet.

If we're in early startup, the impact of a crash is pretty minor - QEMU
will simply exit, which is not significantly differnt from if a setter
handled it "correctly" by setting an Error **errp. A mgmt app that uses
this and finds a setter which crashes will detect the problem pretty
quickly & report bugs.

QEMU is complex enough that we're unlikely to ever find broken setters
by code inspection. So if we don't allow it in preconfig mode, then
I doubt we'll ever find & fix the problems.

So, IMHO, we should allow qom-set and just fix problems as they come
to light, as we would for any other part of QEMU which has plenty of
scope for crashers in rarely tested codepaths.

Regards,
Daniel
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