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

[Cornelia Huck <cohuck@redhat.com>]

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! I don't want something that we don't
support in QEMU lock us into a bad kernel interface, that's just utterly
broken. 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.

If you really think we cannot make changes, I vote for declaring the
current interface legacy and not further developed, and introduce a new,
separate one that doesn't carry all of the baggage that this one
does. We could even end up with a QEMU part that looks better!