* This Week in Spectrum, 2020-W20
@ 2020-05-17 21:56 Alyssa Ross
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From: Alyssa Ross @ 2020-05-17 21:56 UTC (permalink / raw)
To: discuss, devel
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It feels like things are finally starting to come together.
Infrastructure
--------------
Server memory upgraded. We should now be able to handle more than one
person cloning Spectrum's nixpkgs repo at once!
Documentation
-------------
I added a much requested page[1] to the website with a few tasks a
potential contributor might want to look into. More to come here, too!
[1]: https://spectrum-os.org/todo.html
virtio_wl
---------
I wrote a simple guest C program that connected to a host socket over
virtio_wl, and sent some messages back and forth. In doing this, I've
ended up with a nice set of generic functions for interacting with
virtio_wl -- virtio_wl_connect(), virtio_wl_send(), etc. This is nice,
because it allows me to treat virtio_wl inside a guest much like a
regular socket (albeit one that can only be interacted with using a
custom set of functions). Otherwise, I'd have to do ioctl operations
every time (which seems to be what the various virtio_wl-using Chromium
OS components do).
Having a simple test program (rather than integrating directly into
wlroots) gives me a more simple environment with which to test the other
end of this connection.
crosvm
------
I implemented a little Rust server that listens on a Unix stream socket.
When it receives a message (consisting of an identifier {for debug
purposes} and a size), it will allocate a memfd of the requested size,
and then send that back to the socket, along with a status byte
indicating success or failure.
I then integrated this program into crosvm, so it is started as part of
the VM initialization, and exposed over virtio_wl as an additional
socket. Integrating this into crosvm required some careful thought,
because there's no precedent in crosvm for a virtio_wl server of this
sort. It has almost nothing with the existing concept in crosvm of a
"device", which is something that communicates with a guest kernel
driver over PCI. So Spectrum's crosvm now has the concept of
"servers". I can see us wanting to provide other VM services over
virtio_wl in the future. It allows us to exchange various resources
between host and guest (or between guests) without having to write a
kernel driver specific to each task.
This all now works well enough that my program inside the VM can request
a chunk of memory, and receive back something that looks a bit like a
memfd, that can then be sent back to the host over a virtio_wl socket.
It was great to see this working the first time!
One thing crosvm devices do that I would also like to apply to this type
of server is sandboxing. The sandboxing mechanism used in crosvm is
pretty generic (it's really just a thin wrapper around Minijail[2]), so
it should be straightforward to reuse. I just have to understand a bit
better how to use it.
The other crosvm change I still need to make is to put this
functionality behind a command line flag. Most VMs should not be able
to give themselves extra memory this way -- really only the Wayland
compositor should be using it. I should also impose some upper bound on
how much memory is allowed to be allocated this way. Such a limit will
probably get harder to add (from a testing point of view) the longer I
wait.
[2]: https://lwn.net/Articles/700557/
So, at lot of very focused progress this week. From here I plan to
finish up the crosvm stuff, and then integrate this protocol into
wlroots. Then, not too long from now, we'll finally be able to move the
Wayland compositor into its own VM, separate from applications.
Exciting!
There will also probably be a Chromium OS release next week, which will
require some updates in Spectrum. If all goes well, I won't actually be
the one doing that this time. ;)
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