Re: [PATCH 0/8] MUSE: Userspace backed MTD v3

[Miklos Szeredi]

On Mon, Jan 25, 2021 at 12:21 AM Richard Weinberger <richard@xxxxxx> wrote:
>
> I'm happy to announce the first non-RFC version of this patch set.
> Over the xmas holidays I found some time to experiment with various userspace
> implementations of MTDs and gave the kernel side more fine-tuning.
>
> Rationale:
> ----------
>
> When working with flash devices a common task is emulating them to run various
> tests or inspect dumps from real hardware. To achieve that we have plenty of
> emulators in the MTD subsystem: mtdram, block2mtd, nandsim.
>
> Each of them implements an ad-hoc MTD and have various drawbacks.
> Over the last years some developers tried to extend them but these attempts
> often got rejected because they added just more adhoc feature instead of
> addressing overall problems.
>
> MUSE is a novel approach to address the need of advanced MTD emulators.
> Advanced means in this context supporting different (vendor specific) image
> formats, different ways for fault injection (fuzzing) and recoding/replaying
> IOs to emulate power cuts.
>
> The core goal of MUSE is having the complexity on the userspace side and
> only a small MTD driver in kernelspace.
> While playing with different approaches I realized that FUSE offers everything
> we need. So MUSE is a little like CUSE except that it does not implement a
> bare character device but an MTD.

Looks fine.

I do wonder if MUSE should go to drivers/mtd/ instead.   Long term
goal would be move CUSE to drivers/char and move the transport part of
fuse into net/fuse leaving only the actual filesystems (fuse and
virtiofs) under fs/.

But for now just moving the minimal interface needed for MUSE into a
separate header (<net/fuse.h>) would work, I guess.

Do you think that would make sense?

>
> Notes:
> ------
>
> - OOB support is currently limited. Currently MUSE has no support for processing
>   in- and out-band in the same MTD operation. It is good enough to make JFFS2
>   happy. This limitation is because FUSE has no support more than one variable
>   length buffer in a FUSE request.
>   At least I didn’t find a good way to pass more than one buffer to a request.
>   Maybe FUSE folks can correct me. :-)

If you look at fuse_do_ioctl() it does variable length input and
output at the same time.  I guess you need something similar to that.

Thanks,
Miklos