Status of Rust in the block subsystem

I would like to propose a session to discuss the status of Rust in the block subsystem. I have two distinct points to discuss: 1) The status of Rust in the block layer, and 2) the out-of-tree Rust NVMe driver.

== The status of Rust in the block subsystem ==

Rust has special status in the block subsystem [1]. We are currently at "Stage 1" of a two stage "kind of thing", where enabling Rust code in the block subsystem is essentially an experiment that can be terminated if it does not work out. In order to move on from this stage to "Stage 2", where Rust in the block subsystem is a first class citizen, we need two things. 1) We must observe that Rust is applicable for writing block device drivers and 2) the kernel in general must have fully adopted Rust.

For 1) I just sent a patch series that brings the Rust null block driver to feature parity with the C counterpart [2]. Although the null block driver is not real hardware, I think this submission goes a long way to demonstrate that Rust is in fact applicable for writing block device drivers.

For 2) I would point to the fact that Rust in general is no longer experimental in the kernel as a whole [3]. The "Rust Experiment" has been concluded, successfully. We see adoption in the form of the binder driver, and in the large, ongoing, effort targeting GPU drivers in the DRM subsystem. A few network PHY drivers also made it in. There is a lot of activity on the Rust list though, and I expect we will see Rust in more and more subsystems over the next few years.

On that basis, I suggest we discuss whether or not we can move to "Stage 2" - and what that entails.

== The out-of-tree Rust PCI NVMe driver ==

As you may know, I am maintaining a Rust PCI NVMe driver out of tree. For the development work of the block layer Rust APIs, it has been essential to implement a real driver, in addition to the null block driver. A driver targeting actual hardware has to deal with interrupts, registers, IO memory, DMA, memory mappings, in a way that an emulated device does not. Keeping this driver around has helped shape the Rust block APIs in a way that makes sense for real devices, not just emulated ones.

Rebasing this driver every cycle is a lot of work. Driven by requirements of the DRM drivers, core Rust APIs for DMA, register access, IO memory, etc. are being reshaped and rethought, which is great. Rebasing on top of these changes as a second class citizen all the time - not so great. But I believe that the Rust NVMe driver is a great piece of reference code, so I keep doing it.

I would ask that we discuss merging the Rust NVMe driver so as to have a reference implementation of a real driver in-tree. I think it would have the following benefits:

- It would help ensure that the Rust block layer API actually works with real devices.

- It would help make sure that the changes to core APIs that are made to facilitate other subsystems are done in a way that is compatible with block layer requirements.

- It would serve as a nice reference implementation and a great piece of documentation.

- I would not have to rebase it all the time. Patches changing Rust APIs used by Rust NVMe would have to change the Rust NVMe call sites.

Please note the emphasis on *reference* driver. I am not suggesting to replace the existing C driver. I will maintain the new code if necessary.

If we could establish consensus on this, I would clean up the driver to a state where it can be included in the kernel.

Best regards,
Andreas Hindborg