[cycloptic]

>there's no other option for accessing resources

That seems like it would create difficulties in porting software there. Please correct me if I'm wrong but the original plan9 appears to also have no support for shared memory or for poll/select.

>Backfilling IO on page fault is really all mmap does, conceptually.

For read-only resources yes, for handling writes to the mmapped region, that seems quite broken.

[ori_b]

Plan 9 is not a posix system. That means it doesn't have to deal with legacy posix behavior. If you want unix, it's easy to get it.

> For read-only resources yes, for handling writes to the mmapped region, that seems quite broken.

No more broken than mmap of nfs. Consistency is hard.

[cycloptic]

>No more broken than mmap of nfs.

Right, I get that's what you meant, it doesn't seem to really change much versus NFS, or DCOM, or whatever. So it's unclear what benefit is being provided by 9p here.

Also upon further research I am not sure what you mean by this is the only option, plan9 seems to suggest use of channels for other types of IPC interfaces, which seem to not be the same as 9p and are not necessarily network serializable. (Or are they?)

[ori_b]

Channels are not IPC -- they're a libthread API that stays within a shared-memory thread group.

There are few magic kernel devices that don't act like 9p, like '#s' which implements fd passing on a single node. And the VGA drivers expose a special memory segment on PCs to enable configuring VGA devices.

But the exceptions are very few and far in between, and affect very few programs.

> So it's unclear what benefit is being provided by 9p here.

A uniform and simple API for interacting with out-of-process resources that can be implemented in a few hundred lines of code.

[cycloptic]

How is that conceptually different from IPC? The graphics system appears to somehow pass mouse and keyboard events to the client programs over a channel. At least that part seems similar to an Unix X11 setup where this would be done over a socket.

I guess I just don't see what is conceptually the difference here versus something like doing basic HTTP over a TCP socket, it seems like the same kind of multiplexing. Either way, you still have to deal with the same issues: can't pass pointers directly, need to implement byte swapping, need another serialization library if you want the format to be JSON/XML or if you want a schema, etc... So in cases where that stuff isn't important, channels would come in handy, but of course that is now getting closer to a local Unix IPC. Am I getting this right?

[ori_b]

> How is that conceptually different from IPC? The graphics system appears to somehow pass mouse and keyboard events to the client programs over a channel.

A thread reads them from a file descriptor and writes them to a channel. You can look at the code which gets linked into the binary:

    /sys/src/libdraw/mouse.c:61

Essentially, the loop in _ioproc is:

    while(read(fd, event)){
       parse(event);
       send(mousechan, event);
    }

And yes, once you have an open FD, read() and write() act similar to how they would elsewhere. The difference is that there are no OTHER cases. All the code works that way, not just draw events.

And getting the FD is also done via 9p, which means that it naturally respects namespaces and can be interposed. For example, sshnet just mounts itself over /net, and replaces all network calls transparently for all programs in its namespace. Because there's no special case API for opening sockets: it's all 9p.

[cycloptic]

Ok I see, that helps, thank you. That seems to be mostly similar to evdev on Linux after all, except it requires you to use coroutines instead of having an option for a poll/select type interface.

To me the problem with saying "no special cases" seems to make it quite limited on the kernel side and prevent optimization opportunities. For example if you look at the file node vtables on Linux [0] and FreeBSD [1] there are quite a lot of other functions there that don't fit in 9p. So you lose out on all that stuff if you try to fit everything into a 9p server or a FUSE filesystem or something else of that nature.

[0]: https://elixir.bootlin.com/linux/v5.11.8/source/include/linu...

[1]: https://github.com/freebsd/freebsd-src/blob/master/sys/kern/...