Re: [Qemu-devel] [PATCH 4/6] hmp: Add info commands for preconfig

[Markus Armbruster <armbru@redhat.com>]

"Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:

> * Markus Armbruster (armbru@redhat.com) wrote:
>> "Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:
>> 
>> > * Markus Armbruster (armbru@redhat.com) wrote:
>> >> Peter Xu <peterx@redhat.com> writes:
>> >> 
>> >> > On Tue, Jun 05, 2018 at 01:26:34PM +0100, Dr. David Alan Gilbert (git) wrote:
>> >> >> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
>> >> >> 
>> >> >> Allow a bunch of the info commands to be used in preconfig.
>> >> >> Could probably add most of them.
>> >> >
>> >> > I guess some of them may not work yet during preconfig.  E.g.:
>> >> >
>> >> > $ ./x86_64-softmmu/qemu-system-x86_64 -preconfig -monitor stdio
>> >> > QEMU 2.12.50 monitor - type 'help' for more information
>> >> > (qemu) info mtree
>> >> > address-space: memory
>> >> >   0000000000000000-ffffffffffffffff (prio 0, i/o): system
>> >> >
>> >> > address-space: I/O
>> >> >   0000000000000000-000000000000ffff (prio 0, i/o): io
>> >> >
>> >> > But it's fine to enable that I guess.
>> >> >
>> >> > (Which "info" command would you want to use during preconfig?)
>> >> >
>> >> >> 
>> >> >> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>> >> >
>> >> > Reviewed-by: Peter Xu <peterx@redhat.com>
>> >> 
>> >> The reason for having -preconfig is us despairing of making -S do the
>> >> right thing.  We'd have to *understand* the tangled mess that is our
>> >> startup, and rearrange it so QMP becomes available early enough for
>> >> configuring NUMA (and other things), yet late enough for everything to
>> >> work.
>> >> 
>> >> -preconfig is a cheap hack to avoid this headache, by bypassing almost
>> >> all of "everything".
>> >> 
>> >> Now you bring back some of "everything".  Dangerous.  You better show it
>> >> actually works.  Until you do:
>> >> 
>> >> NAK
>> >
>> > Well I did test each command in here to make sure it didn't
>> > crash/produce complete junk; but here's the output with the v2 of this
>> > patch that Igor R-b:
>> [...]
>> 
>> For the sake of the argument, let's assume these commands all work in
>> preconfig state.  Are their QMP equivalents all available in preconfig
>> state?
>
> That I don't know; I was happy to fix my list to the ones
> Igor recommended.  If you object to some particular entries I'll
> be happy to change them.

HMP must not provide more functionality than QMP.  Specifically, we may
provide "info FOO" only when we also provide query-FOO.

There are exceptions to this rule.  I don't think they apply here.  I'm
prepared to discuss them, of course.

I wish there was a way to automate "provide command in HMP when its
buddy is available in QMP", but since the buddies are only connected by
code, that seems infeasible.

Without such automation, the two sets of available commands need to be
kept consistent manually.  The larger they are, the more of a bother.

Bother is fine when it provides commensurate value to users.  Options in
increasing order of value provided:

(1) HMP becomes ready only after we exit preconfig state (what I
    proposed in Message-ID: <87603cxob9.fsf@dusky.pond.sub.org>.

(2) HMP provides help, quit, exit-preconfig.

(3) HMP provides (a subset of) the commands QMP provides.

I figure the maintenance cost of (1) and (2) will be negligible, but (3)
could be a drag.  Are you sure it's worthwhile?