Re: [Qemu-devel] [PATCH v5 1/6] machine: Convert the valid cpu types to use cpu_model

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 6/20/19 4:43 PM, Eduardo Habkost wrote:
> On Thu, Jun 20, 2019 at 11:02:39AM +0200, Igor Mammedov wrote:
>> On Tue, 18 Jun 2019 10:55:16 -0300
>> Eduardo Habkost <ehabkost@redhat.com> wrote:
>>
>>> On Tue, Jun 18, 2019 at 01:34:10PM +0200, Igor Mammedov wrote:
>>>> On Mon, 17 Jun 2019 13:27:00 -0300
>>>> Eduardo Habkost <ehabkost@redhat.com> wrote:
>>>>    
>>>>> On Mon, Jun 17, 2019 at 05:33:43PM +0200, Igor Mammedov wrote:
>>>>>> On Mon, 17 Jun 2019 17:15:21 +0200
>>>>>> Philippe Mathieu-Daudé <philmd@redhat.com> wrote:
>>>>> [...]
>>>>>>> Yes. Eduardo and you should write some lines to explain this, and then
>>>>>>> we will follow :)
>>>>>> Unfortunately I don't recall details anymore. One could check out all
>>>>>> implementations of class_by_name callbacks to find out current state.
>>>>>
>>>>> See this message for a summary of existing class_by_name quirks:
>>>>>
>>>>>    https://www.mail-archive.com/qemu-devel@nongnu.org/msg615503.html
>>>>>    Date: Wed, 08 May 2019 10:34:44 +0200
>>>>>    Message-ID: <877eb173a3.fsf@dusky.pond.sub.org>
>>>>>    Subject: Re: [Qemu-devel] [PATCH 0/7] Delete 16 *_cpu_class_by_name() functions
>>>>>
>>>>> I'm unsure about Igor's suggestion to get rid of CPU model names
>>>>> and use only QOM type names in external interfaces.  In either
>>>>> case, we can still simplify the rules rules and reduce the amount
>>>>> of arch-specific code.
>>>> as far as we have cpu_class_by_name, we have to watch over that
>>>> new patches/targets won't add some custom handling/fallbac/whatnot.
>>>
>>> We can get rid of CPUClass::cpu_class_by_name() without changing
>>> the external interfaces provided by QEMU.
>> if you mean QMP, than it is possible to keep model there where
>> it already exists. Based on experiment [1](x86) I did, it's local to
>> affected target and doesn't pollute other code.
> 
> I mean both command line and QMP.
> 
>>
>>> I don't have a strong opinion about using only QOM types at -cpu,
>>> yet.  But first we need to get rid of the arch-specific CPU model
>>> name exceptions enumerated at the URL above (which would be very
>>> welcome).
>> One way to get rid of them is to deprecate them in favor of strict
>> match (no fallback/substitutions/aliases) to typename and when we
>> drop it make switch type based naming only.
> 
> This is true, but is it desirable?  Aren't we just moving
> complexity elsewhere?  Management software would still need to
> translate CPU models from existing VM configurations to QOM type
> names.
> 
>>
>> 1) I've just took a quick look at how much of duplicated/confusing
>> code we could get rid off if we drop cpu_class_by_name/*_cpu_list.
>> It amounts to >800LOC of trivial removal (not counting ppc/s390
>> that depend on model naming heavily and in need of some non
>> trivial refactoring)
> 
> Removing the code might be trivial.  Coordinating with the
> maintainers of all architectures to confirm this is really
> something everybody wants is the difficult part.
> 
> If you really want to do it, please make sure all the
> architecture maintainers (and libvirt developers) are involved in
> the discussion.

 From the previous link (summary of existing class_by_name quirks):
https://www.mail-archive.com/qemu-devel@nongnu.org/msg615737.html

   "Deprecating unwanted stuff now is likely to make a
    later cleanup so much easier."

This was 8 months ago :/

IIUC we need to restart this topic addressing the libvirt community 
first, then see with the QEMU one?

> 
>>
>>>
>>>>
>>>> On contrary -device works just with type names for all devices,
>>>> applying the same to -cpu which is basically translator
>>>>     model->type[,-global type.foo,...]
>>>> would be consistent with -device and less confusing for everyone
>>>> (not counting significant code reduction).
>>>> It would certainly simplify contributing new targets as contributor
>>>> won't have to care about cpu model naming and do something about it.
>>>>
>>>> This option wasn't considered before because we didn't have deprecation
>>>> back then, but now it opens possibility to simplify qemu and consolidate
>>>> naming. (we probably would be able to fold '-cpu help' into '-device help'
>>>> as well).
>>>>    
>>>
>>
>