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Re: [MirageOS-devel] Rust unikernels

On Thu, Jul 30, 2015 at 5:03 AM, Geoffroy Couprie <contact@xxxxxxxxxxxxxxxxxxx> wrote:
Hi Len,

On Wed, Jul 29, 2015 at 9:12 PM, Len Maxwell <len@xxxxxxxx> wrote:
> Hi Geoffroy and Thomas,
> On Tue, Jul 28, 2015 at 4:29 AM, Thomas Leonard <talex5@xxxxxxxxx> wrote:
>> On 27 July 2015 at 15:54, Thomas Leonard <talex5@xxxxxxxxx> wrote:
>> > On 27 July 2015 at 09:36, Geoffroy Couprie <contact@xxxxxxxxxxxxxxxxxxx>
>> > wrote:
>> [...]
>> One other thing I should mention: Mini-OS runs in kernel mode and can
>> be interrupted at any time. On x86, the interrupt handler uses the
>> same stack as the rest of the code. Therefore, you must compile with
>> the Rust equivalent of -mno-red-zone so Rust doesn't assume it can
>> store things below the current stack pointer.
> I'm just starting to learn Rust, but I managed to create a simple static lib
> and link it into a standalone Mini-OS build:
> * build Rust from git with static musl libc support [1]

This is nice, I did not know about musl usage. This may be a bit big
for a unikernel, though. Do we need a full POSIX interface, with
threads and everything?


OK, now I see that targeting musl-static brings in too much additional/unnecessary code; that was my misguided way of working around linker errors between libhello and mini-os. Your "mm" package imports the "libc" crate; I suppose it adds the Rust stubs for libc functions, which are then linked to the implementations in mini-os? (which, in turn, are not needed for the trivial Rust code)

It looks like mirage-platform/bindings [a] captures most of the interface between Mirage/OCaml and Mini-OS. It uses a few functions from time.h, string.h, and various Xen-related APIs. There's also main.c [b] which overrides start_kernel to kick off the OCaml runtime. Not sure if any additional functions are needed by OCaml itself?

Rust and OCaml also have a common dependency on libm. For Mirage on Xen, it is pulled in via mirage-platform/xen-posix.

Thomas, what's mirage take on this?

> * compile libhello.a from [2]
Those should be the correct compilation flags

> * declare the Rust function as an extern in mini-os/kernel.c, call it from
> start_kernel()
> * add -L... -lhello to the Mini-OS linker flags
> * make and run
> Geoffroy, how does this compare to your approach?

See my code here: https://github.com/Geal/mini-os/compare/master...rust

Basically, I'm trying to replace some parts of Mini-OS with Rust code,
as Thomas suggested. That way, we keep an architecture and a
bootloader that work well with Xen, and it can be tested right away
with Mirage.

> To understand the interactions between Xen, Mini-OS, and MirageOS, I found
> [3] and [4] useful starting points.
We will need a Rust implementation of clients for Xenbus and Xen
events. Once code is running from start_kernel, the next step is
interacting with Xen's devices.

I've started a learning exercise to see how much I can delete from Mini-OS while still supporting Mirage. Threading, POSIX emulation, HAVE_LIBC, and all of the front/back devs can go (except console), and then I might actually understand what's left.

> The zinc.rs [5] project, a Rust RTOS for ARM, could provide some inspiration
> for modeling low-level operations in a type-safe way.

Zinc looks great, and they do something that I was planning to:
downloading the libcore part of Rust and linking to it directly. This
is the easiest way to get Rust features as needed, without having too
many dependencies. And once there's a libcore, making a libstd gets

Not knowing much about libcore vs. libstd -- which parts of libstd are important to the task of replacing C code in Mini-OS?

Speaking of ARM, it would be interesting to see if the init_mm experiment runs on a Cubie:Âhttps://mirage.io/blog/introducing-xen-minios-arm

> [1]: https://doc.rust-lang.org/nightly/book/advanced-linking.html#linux
> [2]: https://gist.github.com/lnmx/e7a74bacafb7e38b8355
> [3]: http://roscidus.com/blog/blog/2014/07/28/my-first-unikernel/
> [4]: https://mirage.io/wiki/xen-events
> [5]: http://zinc.rs

Geoffroy Couprie


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