Re: [PATCH V2] migration: file URI

[Peter Xu]

On Wed, Jun 14, 2023 at 02:59:54PM -0300, Fabiano Rosas wrote:
> In this message Daniel mentions virDomainSnapshotXXX which would benefit
> from using the same "file" migration, but being done live:
> 
> https://lore.kernel.org/r/ZD7MRGQ+4QsDBtKR@redhat.com
> 
> And from your response here:
>  https://lore.kernel.org/r/ZEA759BSs75ldW6Y@x1n
> 
> I had understood that having a new SUSPEND cap to decide whether to do
> it live or non-live would be enough to cover all use-cases.

Oh, I probably lost some of the contexts there, sorry about that - so it's
about not being able to live snapshot on !LINUX worlds properly, am I
right?

In the ideal world where we can always synchronously tracking guest pages
(like what we do with userfaultfd wr-protections on modern Linux), the
!SUSPEND case should always be covered by CAP_BACKGROUND_SNAPSHOT already
in a more performant way.  IOW, !SUSPEND seems to be not useful to Linux,
because whenever we want to set !SUSPEND we should just use BG_SNAPSHOT.

But I think indeed the live snapshot support is not good enough. Even on
Linux, it lacks different memory type supports, multi-process support, and
also no-go on very old kernels.  So I assume the fallback makes sense, and
then we can't always rely on that.

Then I agree we can keep "file:" the same as others like proposed here, but
I'd like to double check with all of us so we're on the same page..  And
maybe we should mention some discussions into commit message or comments
where proper in the code, so we can track what has happened easier.

Thanks,

-- 
Peter Xu