On Tue, Dec 07 2021, Jason Gunthorpe <jgg@xxxxxxxxxx> wrote: > On Tue, Dec 07, 2021 at 11:50:47AM +0100, Cornelia Huck wrote: >> On Mon, Dec 06 2021, Jason Gunthorpe <jgg@xxxxxxxxxx> 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 :/
