Re: Redesign of QEMU startup & initial configuration

[Mark Burton]

> On 14 Dec 2021, at 12:48, Markus Armbruster <armbru@redhat.com> wrote:
> 
> Paolo Bonzini <pbonzini@redhat.com> writes:
> 
>> On 12/13/21 16:28, Markus Armbruster wrote:
>>> Paolo Bonzini <pbonzini@redhat.com> writes:
>>> 
>>>> On 12/10/21 14:54, Markus Armbruster wrote:
>>>>> I want an open path to a single binary.  Taking years to get there is
>>>>> fine.
>>>> 
>>>> The single binary is a distraction in my opinion.  Imagine
>>>> instead of vl.c you have this in your second binary:
> 
> [...]
> 
>>>> This is the ultimate QEMU startup code.  If we can get this code to
>>>> actually build a machine, you've reached the point where you don't care
>>>> about what is in the command line parser; and consequently you don't care
>>>> if there is one binary or two.
>>> 
>>> Define "you".  Also explain why it should include me, because I think it
>>> doesn't :)
>> 
>> Impersonal you. :)
> 
> Unfortunate choice of a word.
> 
>>> By when can we have this second binary in master?  Opinion, please, not
>>> promise.
>> 
>> Define "have":
>> 
>> - a binary that builds
>> 
>> - a binary that builds a bootable guest
>> 
>> - a binary that builds any guest that the current well-maintained
>>  targets can build, using a given (but roughly full-featured) subset
>> of options
>> 
>> Estimates for the first are easy (it's in my tree), estimates for the
>> second depends on somebody helping (upstreaming -M smp took months 
>> between me being busy, reviewers being busy, and releases freezing
>> development), estimates for the third are hard.
> 
> Thanks.
> 
>>> Would you object to me expanding the CLI here to the point where I think
>>> we can deprecate the old binary?
>>> 
>>> If yes, why?
>> 
>> Yes, for two reasons.
>> 
>> First, because there will be usually differences between the command
>> lines as mentioned elsewhere in the thread.  qemu-system-* is a good 
>> name, but one that is already taken by 15 years of docs using the
>> existing command line.
> 
> A new CLI is pointless unless there are differences to the old one.
> 
> It is unadvisable unless we can eventually retire the old one.
> 
> While they coexist, the old binary name should use the old CLI, to
> reduce confusion.
> 
>> Second, because a command line is really hard to get right as
>> complexity increases.  QMP is the way to go to get as clean as
>> possible a configuration mechanism.  There *will* be a second set of
>> warts layered on top of the above code, and I don't want that.
> 
> We do not have consensus.  We may have misunderstandings.
> 
> Let's start with where we (hopefully) agree:
> 
> * 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.
> 
> * 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.

For “Completing” the interface, I agree. 
To add a certain number of use cases - many of those can be (have been - aka 
preconfig) done, if with some degree of unpleasant-ness NOW without full 
re-working. That would give us test cases that we can subsequently use to test 
against as we move forward.

> 
> * A new binary sidesteps the need to manage incompatible change.
> 
> 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.
> 
> * We already layer an interface on top: HMP[2].  It has its warts.
> 
> * The old CLI is partly layered on QMP, partly on HMP, and partly on
>  internal C interfaces.  It's full of warts.
> 
> * Management applications are not the only users that matter.  Humans
>  matter.  Simple programs like ad hoc scripts matter.
> 
(Unless one considers that a ‘human’ and/or ’script’ interface would just be 
‘yet another management interface’…. And can/should be relegated to Somebody 
Else’s Problem)

Cheers
Mark.


> Objections to these?
> 
> [...]
> 
> 
> [1] Actually a QEMU character device now, but let's ignore that.
> 
> [2] Except where we choose to bypass QMP, but that's unimportant here.
>