Re: [Qemu-arm] [Qemu-devel] [PATCH 9/9] hw/arm/virt: Add virt-3.0 machine type

[Auger Eric]

Hi Dan, Laszlo,

On 06/14/2018 10:59 AM, Daniel P. Berrangé wrote:
> On Thu, Jun 14, 2018 at 10:56:20AM +0200, Laszlo Ersek wrote:
>> Hi Eric,
>>
>> On 06/14/18 08:27, Auger Eric wrote:
>>> Hi Laszlo,
>>>
>>> On 06/13/2018 11:05 PM, Laszlo Ersek wrote:
>>>> On 06/13/18 10:48, Eric Auger wrote:
>>>>
>>>>> PATCH: merge of ECAM and VCPU extension
>>>>> - Laszlo reviewed the ECAM changes but I dropped his R-b
>>>>>   due to the squash
>>>>
>>>> Was there any particular reason why the previous patch set (with only
>>>> the ECAM enlargement) couldn't be merged first? To be honest I'm not
>>>> super happy when my R-b is dropped for non-technical reasons; it seems
>>>> like wasted work for both of us.
>>>>
>>>> Obviously if there's a technical dependency or some other reason why
>>>> committing the ECAM enlargement in separation would be *wrong*, that's
>>>> different. Even in that case, wouldn't it be possible to keep the
>>>> initial virt-3.0 machtype addition as I reviewed it, and then add the
>>>> rest in an incremental patch?
>>>
>>> Sorry about that. My fear was about migration. We would have had 2 virt
>>> 3.0 machine models not supporting the same features. While bisecting
>>> migration we could have had the source using the high mem ECAM and the
>>> destination not supporting it. So I preferred to avoid this trouble by
>>> merging the 2 features in one patch. However I may have kept your R-b
>>> restricting its scope to the ECAM stuff.
>>
>> to my understanding, it is normal to *gradually* add new properties
>> during the development cycle, to the new machine type of the upcoming
>> QEMU release. To my understanding, it's not expected that migration work
>> between development snapshots built from git. What matters is that two
>> official releases, specifying the same machine type, enable the user to
>> migrate a guest between them (in forward direction).
>>
>> In every release, so many new features are introduced that it's
>> impossible to introduce the new machine type with all the compat knobs
>> added at once. Instead, the new machine type is introduced when the
>> first feature that requires a compat knob is added to git. All other
>> such features extend the compat knobs gradually, during the development
>> cycle. Until the new official release is made (which contains all the
>> compat knobs for all the new features), the new machine type simply
>> doesn't exist, as far as the public is concerned, so it cannot partake
>> in migration either.
>>
>> This is my understanding anyway.
> 
> That is correct - there is ZERO expectation of migration / ABI stability
> between arbitrary GIT snapshots, only official releases.  Prior to the
> first release including it, a versioned machine type can be changed
> arbitrarily.
OK so sufficient consensus on this then.

Thanks

Eric
> 
> Regards,
> Daniel
>