Re: [PATCH v2 10/10] hmp: Deprecate 'singlestep' member of StatusInfo

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

* Peter Maydell (peter.maydell@linaro.org) wrote:
> On Tue, 4 Apr 2023 at 14:25, Markus Armbruster <armbru@redhat.com> wrote:
> >
> > Peter Maydell <peter.maydell@linaro.org> writes:
> >
> > > On Tue, 4 Apr 2023 at 09:25, Markus Armbruster <armbru@redhat.com> wrote:
> > >> Hmm.  We report it in query-status, which means it's relevant to QMP
> > >> clients.  We provide the command to control it only in HMP, which means
> > >> it's not relevant to QMP clients.
> > >>
> > >> Why is reading it relevant to QMP clients, but not writing?
> > >
> > > I suspect that neither is very relevant to QMP clients, but I
> > > thought we had a requirement that HMP interfaces went
> > > via QMP ones ?
> >
> > Kind of.  Here's my current boilerplate on the subject:
> >
> >     HMP commands without a QMP equivalent are okay if their
> >     functionality makes no sense in QMP, or is of use only for human
> >     users.
> >
> >     Example for "makes no sense in QMP": setting the current CPU,
> >     because a QMP monitor doesn't have a current CPU.
> >
> >     Examples for "is of use only for human users": HMP command "help",
> >     the integrated pocket calculator.
> >
> >     Debugging commands are kind of borderline.  Debugging is commonly a
> >     human activity, where HMP is just fine.  However, humans create
> >     tools to assist with their activities, and then QMP is useful.
> >     While I wouldn't encourage HMP-only for the debugging use case, I
> >     wouldn't veto it.
> >
> > When adding an HMP-only command, explain why it is HMP-only in the
> > commit message.
> >
> > >                If not, we could just make the HMP query
> > > interface directly look at the TCG property, the way the
> > > write interface does.
> >
> > How useful is it HMP?
> 
> Well, as usual, we have no idea if anybody really uses any feature.
> I've never used it myself but I have a vague recollection of reading
> list mail once from somebody who used it. You can construct theoretical
> scenarios where it might be nice (eg "boot guest OS quickly and then
> turn on the one-insn-per-tb mode once you get to the point of interest"),
> I guess. These theoretical scenarios are equally valid (or esoteric)
> whether you're trying to control QEMU via QMP or HMP.
> 
> I think on balance I would go for:
>  * remove (ie deprecate-and-drop) 'singlestep' from the QMP struct,
>    rather than merely renaming it
>  * if anybody comes along and says they want to do this via QMP,
>    implement Paolo's idea of putting the accelerator object
>    somewhere they can get at it and use qom-get/qom-set on it
>    [My guess is this is very unlikely: nobody's complained so
>    far that QMP doesn't permit setting 'singlestep'; and
>    wanting read without write seems even more marginal.]
>  * keep the HMP commands, but have both read and write directly
>    talk to the accel object. I favour splitting the 'read'
>    part out into its own 'info one-insn-per-tb', for consistency
>    (then 'info status' matches the QMP query-status)

If it's pretty obscure, then the qom-set/get is fine; as long
as there is a way to do it, then just make sure in the commit
message you say what the replacement command is.

Dave

> In particular, the fact that messing with this obscure debug
> functionality requires updating the reference-output for a
> bunch of io tests that have no interest at all in it rather
> suggests that even if we did want to expose this to QMP that
> the query-status command is the wrong place to do it.
> 
> thanks
> -- PMM
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK