Re: Redesign of QEMU startup & initial configuration

[Mark Burton]

FWIW I Agree.

(Which probably means somethings hiding somewhere :-) )

Cheers
Mark.

> On 15 Dec 2021, at 21:00, Paolo Bonzini <pbonzini@redhat.com> wrote:
> 
> On 12/14/21 12:48, Markus Armbruster wrote:
>> Let's start with where we (hopefully) agree:
> 
> More or less I do agree with this, except for a couple points below where I 
> think we disagree.
> 
>> * We need a single, cohesive, low-level interface suitable for
>>   management applications.
>> * The existing interface is specified in QAPI.  Its concrete transport
>>   is QMP.
>> * The existing interface is not complete: certain things can only be
>>   done with the CLI.
>> 
>> * The existing transport is not available early enough to permit
>>   completing the interface.
> 
> So far so good.
> 
>> * Fixing that involves a rework of startup.
>> * Reworking the existing startup and managing incompatible changes is
>>   impractical, and likely to make the mess we have on our hands worse.
> 
> Not really, in particular the startup has been mostly reworked already and I 
> disagree that it is messy.  softmmu/vl.c is messy, sure: it has N different 
> command line parser for command line options, magic related to default 
> devices, and complicated ordering of -object creation.
> 
> But the building of emulator data structures is not messy; only the code that 
> transforms the user's instructions into startup commands.  The messy parts 
> are almost entirely contained within softmmu/vl.c.
> 
> The one (and important, but fixable) exception is backends for on-board 
> devices: serial_hd, drive_get, and nd_table.
> 
>> * A new binary sidesteps the need to manage incompatible change.
> 
> More precisely, a new binary sidesteps the need to integrate an existing 
> mechanism with a new transport, and deal with the incompatibilities that 
> arise.
> 
>> Any objections so far?
>> Now let me make a few more points:
>> * Single, cohesive interface does not require single transport.  In
>>   fact, we already have two: QMP and the (internal) C interface.
>> * QMP encodes the abstract interface in JSON, and offers the result on a
>>   Unix domain socket[1].
>> * The (internal) C interface encodes the abstract interface as a set of
>>   C data types and functions.
>> * Consider a configuration file transport that encodes the abstract
>>   interface in JSON.  The only wart this adds is syntax that is
>>   arguiably ill-suited to the purpose.  More suitable syntax exists.
>> * Similar for CLI.
>> * To get a "a second set of warts layered on top", we actually have to
>>   layer something on top that isn't utterly trivial.  Like a
>>   higher-level interface.  The "second set of warts" objection does not
>>   apply to (sane) transports.
> 
> The problem is that CLI and HMP, being targeted to humans (and as you say 
> below humans matter), are not necessarily trivial transports.  If people find 
> the trivial transport unusable, we will not be able to retire the old CLI.
> 
> Bad CLI is also very hard to deprecate because, unlike QMP (for which you can 
> delegate the workarounds to Libvirt & friends) and HMP (for which people can 
> just learn the new thing and type it), it is baked in countless scripts.  
> People hate it when scripts break.
> 
>> * The old CLI is partly layered on QMP, partly on HMP, and partly on
>>   internal C interfaces.  It's full of warts.
> 
> I've worked on moving it more towards QMP or at least QOM, and much less on 
> internal C interfaces.  It still has warts but they are self-contained and 
> due to the baroque ordering of options.  My point is that we can continue 
> this work to the point that having separate entry points (one CLI-centered, 
> one QMP-only) is not a problem.
> 
> The issues with the CLI then can be completely self-contained within 
> softmmu/vl.c, and will not influence the innards of QEMU.
> 
> Paolo
> 
>> * Management applications are not the only users that matter.  Humans
>>   matter.  Simple programs like ad hoc scripts matter.