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

[Auger Eric]

Hi Laszlo,
On 06/14/2018 10:56 AM, 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.

Thank you for sharing your understanding. Maybe my concerns were
superficial indeed. If Peter confirms there is no concern with
bisection, I can easily repost the series splitting the virt machine
model modifications in several patches, keeping your R-b ;-)

Thanks

Eric
> 
> Thanks!
> Laszlo
>