Re: [PATCH] fw_cfg: Allow reboot-timeout=-1 again

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

* Han Han (hhan@redhat.com) wrote:
> However, another important question is: how can we avoid such undocumented
> incompatibility appears again?

The reboot-timeout one was accidental - it was a documented qemu
feature; just no one noticed it when the input check was added.
Officially if we actually want to deprecate a feature we should actually
follow qemu's deprecation guidelines.

> I can show another case caused by such incompatibile change:
> https://bugzilla.redhat.com/show_bug.cgi?id=1745868#c0
> 
> For the qemu devices, attributes, values, qmp cmds, qmp cmds arguments used
> by libvirt, could we get a way to inform libvirt
> that an incompatibile qemu change is coming, please update libvirt code
> ASAP to adjust to that change?
> Or another way that is more gently:  popping up the warning of depreciation
> instead of  dropping it, and then drop it in the version
> after next version.

Yes that should happen;  with deprecated devices it's easier than more
subtle features like command line things;  I'm not sure how that gets
introspected.  I thought libvirt already asked qemu for a list of
devices, so I'm confused why libvirt didn't spot it before starting the
VM in 1745868.

Dave

> 
> On Tue, Oct 29, 2019 at 1:59 PM Dr. David Alan Gilbert <dgilbert@redhat.com>
> wrote:
> 
> > * Markus Armbruster (armbru@redhat.com) wrote:
> > > "Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:
> > >
> > > > * Markus Armbruster (armbru@redhat.com) wrote:
> > > >> "Dr. David Alan Gilbert (git)" <dgilbert@redhat.com> writes:
> > > >>
> > > >> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > > >> >
> > > >> > Commit ee5d0f89de3e53cdb0dc added range checking on reboot-timeout
> > > >> > to only allow the range 0..65535; however both qemu and libvirt
> > document
> > > >> > the special value -1  to mean don't reboot.
> > > >> > Allow it again.
> > > >> >
> > > >> > Fixes: ee5d0f89de3e53cdb0dc ("fw_cfg: Fix -boot reboot-timeout
> > error checking")
> > > >> > RH bz: https://bugzilla.redhat.com/show_bug.cgi?id=1765443
> > > >> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > >> > ---
> > > >> >  hw/nvram/fw_cfg.c | 5 +++--
> > > >> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > > >> >
> > > >> > diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
> > > >> > index 7dc3ac378e..1a9ec44232 100644
> > > >> > --- a/hw/nvram/fw_cfg.c
> > > >> > +++ b/hw/nvram/fw_cfg.c
> > > >> > @@ -247,10 +247,11 @@ static void fw_cfg_reboot(FWCfgState *s)
> > > >> >
> > > >> >      if (reboot_timeout) {
> > > >> >          rt_val = qemu_opt_get_number(opts, "reboot-timeout", -1);
> > > >> > +
> > > >> >          /* validate the input */
> > > >> > -        if (rt_val < 0 || rt_val > 0xffff) {
> > > >> > +        if (rt_val < -1 || rt_val > 0xffff) {
> > > >> >              error_report("reboot timeout is invalid,"
> > > >> > -                         "it should be a value between 0 and
> > 65535");
> > > >> > +                         "it should be a value between -1 and
> > 65535");
> > > >> >              exit(1);
> > > >> >          }
> > > >> >      }
> > > >>
> > > >> Semantic conflict with "PATCH] qemu-options.hx: Update for
> > > >> reboot-timeout parameter", Message-Id:
> > > >> <20191015151451.727323-1-hhan@redhat.com>.
> > > >
> > > > Thanks for spotting that.
> > > > I think Han and also submitted patches to review it from libvirt
> > > > and it wasn't obvious what to do.  (Cc'd Han in).
> > > >
> > > >> I'm too tired right now to risk an opinion on which one we want.
> > > >
> > > > As is everyone else !  The problem here is that its documented
> > > > as a valid thing to do, and libvirt does it, and you might have
> > > > a current XML file that did it.  Now I think you could change libvirt
> > > > to omit the reboot-timeout parameter if it was called with -1.
> > > >
> > > > So given its a documented thing in both qemu and libvirt xml
> > > > if we want to remove it then it sohuld be deprecated properly - but
> > it's
> > > > already broken.
> > >
> > > Since commit ee5d0f89d, v4.0.0.
> > >
> > > If that commit had not made it into a release, we'd certainly treat the
> > > loss of "-1 means don't reboot" as regression.
> > >
> > > But it has.  We can treat it as a regression anyway.  We can also
> > > declare "ship has sailed".
> > >
> > > I'm leaning towads the former.
> > >
> > > If we restore "-1 means don't reboot", then I don't see a need to
> > > deprecate it.  Just keep it.
> > >
> > > What do you think?
> >
> > That's also my view; especially since the problem seems to be an easy
> > fix.
> >
> > Dave
> >
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> >
> 
> 
> -- 
> Best regards,
> -----------------------------------
> Han Han
> Quality Engineer
> Redhat.
> 
> Email: hhan@redhat.com
> Phone: +861065339333
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK