Re: [PATCH RFC v2] vfio: Documentation for the migration region

[Cornelia Huck <cohuck@redhat.com>]

On Tue, Dec 07 2021, Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Tue, Dec 07, 2021 at 11:50:47AM +0100, Cornelia Huck wrote:
>> On Mon, Dec 06 2021, Jason Gunthorpe <jgg@nvidia.com> wrote:
>> 
>> > On Fri, Dec 03, 2021 at 11:06:19AM -0700, Alex Williamson wrote:
>> 
>> >> This is exactly the sort of "designed for QEMU implementation"
>> >> inter-operability that I want to avoid.  It doesn't take much of a
>> >> crystal ball to guess that gratuitous and redundant device resets
>> >> slow VM instantiation and are a likely target for optimization.
>> >
>> > Sorry, but Linus's "don't break userspace" forces us to this world.
>> >
>> > It does not matter what is written in text files, only what userspace
>> > actually does and the kernel must accommodate existing userspace going
>> > forward. So once released qemu forms some definitive spec and the
>> > guardrails that limit what we can do going forward.
>> 
>> But QEMU support is *experimental*, i.e. if it breaks, you get to keep
>> the pieces, things may change in incompatible ways. And it is
>> experimental for good reason!
>
> And we can probably make an breakage exception for this existing
> experimental qemu.
>
> My point was going forward, once we userspace starts to become
> deployed, it doesn't matter what we write in these text files and
> comments. It only matters what deployed userspace actually does.

Absolutely, as soon as QEMU support is made non-experimental, this needs
to be stable.

>
>> It would mean that we must never introduce experimental interfaces
>> in QEMU that may need some rework of the kernel interface, but need
>> to keep those out of the tree -- and that can't be in the best
>> interest of implementing things requiring interaction between the
>> kernel and QEMU.
>
> In general we should not be merging uAPI to the kernel that is so
> incomplete as to be unusable. 
>
> I'm sorry, this whole thing from the day the migration stuff was first
> merged to include/uapi is a textbook example how not to do things in
> the kernel community.

Honestly, I regret ever acking this and the QEMU part. Maybe the history
of this and the archived discussions are instructive to others so that
they don't repeat this :/