Re: Redesign of QEMU startup & initial configuration

[Markus Armbruster]

Mark Burton <mark.burton@greensocs.com> writes:

>> 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.

I'd be okay with hacking up the current mess some more so it can serve
as a test bed, as long as we all understand that the hacks are to be
reverted.

>> * 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)

I really hate relying on this Somebody guy, he never gets anything done.

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