Re: Development on the Spectrum host

[Ville Ilvonen <ville.ilvonen@unikie.com>]

On 18.8.2022 13.17, Alyssa Ross wrote:

> Okay.  Obviously we can support booting with the filesystem mounted
> read-write and no dm-verity.  But the problem with that is that then
> changes have to be manually copied back to the source tree.  We could

One  way to handle this is so that people git clone to the target and 
build small incremental changes there.

> try to add some conveniences for that (e.g. a facility to diff the
> modified root filesystem with the orginal version).  It still means
> making changes to the compiled system, rather than working on the
> source, but for the kind of changes you're talking about, maybe doing
> it that way and then integrating the changes into the Spectrum source
> at the end wouldn't be too bad?

When installation of development tools to the target is limited, 
remotely mounting filesystem (e.g. sshfs) from the development host works.

> Ideally it would be nice to have something like nixos-rebuild switch,
> that can build a new system and then switch into it.  But the problem
> is, you have to figure out how to *automatically* take the runtime state
> of the system (the processes and services that are running), and somehow
> carry that over into the new system, where anything could have changed.
> So I'm not sure how we'd do that.  NixOS's implementation of this is
> massive, tightly coupled to both NixOS and systemd, and still it's not
> very difficult to get it into a weird state where you need to reboot
> anyway to get an accurate idea of what the change does (or, if you
> don't realise you need to do that, you just think the change hasn't
> worked).  If a kernel module has changed, do you try to unload an > reload it?  That's pretty complicated to implement, because you have to
> unbind the devices from it, unload the module, then rebind them, and
> even then the module might have had important state that's been lost.

Good examples but there's no one answer to these scenarios. Depends 
really much on the case and developer preferences. Often services 
restarting is enough, sometimes system reboot is required. Also, people 
doing kernel development are usually fairly seasoned and consider which 
makes sense to themselves. E.g. kernel modules run reset for the 
hardware in init() and scripts/tests can be used to handle 
unbind-unload-rebind in driver development. Managing state persistence 
over resets or SW updates is an eternity problem. It's an interesting 
problem - I studied dynamic software updates and state migration is one 
key challenge there. DSU has not taken traction, though. It's tough to 
update both code and the runtime state. We have legacy and culture of 
restarts.

In critical systems it's designed and handled with hardware, even system 
redundancy - not at single component level.

> If the developer is working on that kernel module it's probably okay,
> but what if they're working on something else and just pulled a bunch of
> changes that happened to include a kernel module change?  Same with if
> e.g. the Weston service changes, except in that case we have no way to
> restore the previous state.  So either we choose not to automatically
> restart Weston, in which case we're not really running the new system,
> but a weird hybrid that may have bugs not present in either the old or
> new system, or we do restart Weston, and risk an unsuspecting user
> losing their whole windowing session.

True. In these scenarios I've learned not to try to cover every possible 
scenario on behalf of other people. Developers are smart and they 
understand and learn constraints of system changes.

> If there's a way we could make this work well, I'm open to it, but it's
> not at all obvious to me what that would look like.

Given above, I try to keep this at high level design.
Two configurations:
a. development - enables development, testing and debugging with caveats
b. user - immutable, hardened, strong security promises

Now we have b. which makes a. by design impossible *on target*, making 
development iterations slow. The question is if we want to generally 
enable a. and then say - "there it is for anyone who needs it".

Best, -Ville