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

[Laszlo Ersek <lersek@redhat.com>]

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.

Thanks!
Laszlo