Re: [PATCH RFC v2 6/6] KVM: introduce a new API for getting dirty bitmaps

[Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>]

(2010/04/20 20:33), Alexander Graf wrote:
>>>>
>>>> -#define KVM_API_VERSION 12
>>>> +#define KVM_API_VERSION 13
>>>
>>> Is there a way to keep both interfaces around for some time at least? I'd prefer the API version not to change if not _really_ necessary.
>>>
>>> To enable the new dirty mapping you could for example use KVM_CAP_ENABLE_CAP :-).
>>
>> Thanks, I did not know what is the appropriate way for this kind of change.
>>
>> I just read the comments in the kvm.h and thought I had to update the number whenever
>> I added some entries to kvm.h .
>
> The rule of thumb is:
>
> KVM_API_VERSION change ->  complete breakage. No way to run older userspace on newer kernels.
> new CAP ->  optional feature
>
> If I read correctly you're changing quite a few existing interfaces, so old userspace wouldn't work anymore. Hence I'm proposing using activating the new way of dirty logging optionally.
>
> Alex
>

The fact is, I am trying to keep the existing interfaces completely(KVM_API_VERSION part is my mistake).

For x86, I have checked that current qemu works without any change.
I mean we can use get_dirty_log() as is.

And as you propose, we can determine which api(swith_* or get_*) we use optionally.