Re: [Qemu-devel] [libvirt] [qemu RFC v2] qapi: add "firmware.json"

[Laszlo Ersek <lersek@redhat.com>]

On 04/20/18 11:34, Daniel P. Berrangé wrote:
> On Fri, Apr 20, 2018 at 10:11:08AM +0200, Laszlo Ersek wrote:
>> On 04/19/18 11:12, Daniel P. Berrangé wrote:
>>> On Thu, Apr 19, 2018 at 10:39:32AM +0200, Laszlo Ersek wrote:
>>>> On 04/19/18 09:56, Daniel P. Berrangé wrote:
>>>>> On Thu, Apr 19, 2018 at 09:48:36AM +0200, Markus Armbruster wrote:
>>>>>> Laszlo Ersek <lersek@redhat.com> writes:
>>>>>>
>>>>>>> On 04/18/18 10:47, Markus Armbruster wrote:
>>>>>>>> Laszlo Ersek <lersek@redhat.com> writes:
>>>>>> Replacing CpuInfoArch by such an enum will change the discriminator
>>>>>> value from "other" to the real architecture, with the obvious
>>>>>> compatibility concerns.  But we've accepted similar changes twice
>>>>>> already: commit 9d0306dfdfb and commit 25fa194b7b1, both v2.12.0-rc0.
>>>>>>
>>>>>> "other" was a bad idea.  Hindsight 20/20.
>>>>>>
>>>>>> Getting rid of it in one go rather than piecemeal seems like the least
>>>>>> bad way out.  Too late for 2.12, though.  Eric, what do you think?
>>>>>
>>>>> Given the context in which this "other" value is used, I think it is
>>>>> reasonable to kill it and put a full arch list in there.
>>>>>
>>>>> No app is likely to be accessing the struct under "other" because it
>>>>> is just an empty placeholder.
>>>>
>>>> Commit 9d0306dfdfb added "s390" and "CpuInfoS390", which I guess had the
>>>> potential to confuse QMP clients that didn't expect "s390", but
>>>> otherwise it didn't mess with preexistent enum values / structures.
>>>
>>> NB, qemu-system-s390x would previously have returned "other" in
>>> this field, and now it returns "s390".  So while it didn't
>>> remove "other" from the list of things that could potentially
>>> exist, it did change what the s390x binary will actually report.
>>>
>>>> The same applies to commit 25fa194b7b1, just with "riscv" /
>>>> "CpuInfoRISCV" substituted.
>>>>
>>>> Removing "other" might confuse QMP clients that expect it, except
>>>> (according to Daniel) no such client exists, probably.
>>>
>>> When I say removing "other", I imply that we add an explicit arch
>>> for all those which we currently are missing. IOW, all qemu-system-XXX
>>> binaries which currently report "other" would change to report their
>>> respective "XXX" values.
>>>
>>> So in this way, it is exactly the same as what we did when we
>>> introduced "s390" as an option.
>>>
>>> The only difference is that once we have every binary reporting the
>>> correct arch, we can now also remove "other" from the schema itself
>>> as it will then be unused.
>>
>> Can we please translate this into more actionable items for me, because
>> I'm getting confused :)
>>
>> First, if I add "i386" and "x86_64" to the enum list, we'll have all
>> three of "i386", "x86_64" and "x86". Is that useful? How will that work?
> 
> Hmm, yes, on closer look this is a big mess as it is. We've been using
> generic terms for covering multiple architectures :-(  'x86' for both
> i386 and x86_64,  'sparc' for sparc and sparc64, etc. If we try to fix
> that we'll be entering a world of backcompat hurt :-(
> 
> Since your schema is likely to end up just being a file in docs/specs,
> rather than directly part of our existnig qapi schema, I suggest we just
> ignore whats there. Just define an arch enum in your spec which is right,
> and let someone else worry about fixing the mess

I can't tell you how much I love this idea. :)

Thanks!
Laszlo

> 
>> Second, assuming I add constants for the ~10 (?) softmmu arches, can I
>> still use @CpuInfoOther as the type for the corresponding new members in
>> @CpuInfo? What C code changes will be necessary?
> 
> Yes, we could still use the CpuInfoOther struct, since struct names are
> invisible to consumers, but as above, lets ignore the mess
> 
> Regards,
> Daniel
>