Add logging syscall

[m-mueller678]

It would be nice to be able to emit logs from the application in the same style the kernel uses.

I have implemented a logging syscall for this purpose:

pub unsafe extern "C" fn sys_log(
       level: usize,
       target_ptr: *const u8,
       target_len: usize,
       message_ptr: *const u8,
       message_len: usize,
)

Log filtering is handled by the application to enable compile time log filtering.

If desired, I'll open a PR to upstream this.

[mkroening]

I don't think it is worth the effort to introduce, commit to, and maintain a system call for this.

Why can't the application format any logs itself and print them to stderr? This is also how it is usually done in other operating systems.

[m-mueller678]

It is entirely possible to do this in the application. I wanted to avoid just copying the code from the kernel to my app. The downside with that is that if the kernel message format changes, the application needs to update its format to match (or live with having a different format).

I frequently move code between kernel space and user space, so I find it irritating to have different log formats for both. For me, one of the big appeals of a unikernel is being able to integrate tightly with the kernel, so a lot of my application code lives in the kernel. Based on this, I think a desire for integrating application logs into the kernel log would be somewhat common among users, though I may just have a special use case.

[mkroening]

I see. Personally, I see the log of the application and the log of the kernel as something semantically different and would like to keep them separate. The fact that both reach the end user via the same serial interface is unfortunate.

I'll discuss this with the team, though. Maybe they see it differently. If we decide in favor of allowing integration with kernel logging, I think something similar to syslog (syslog.h) would be more appropriate, as that is something that other applications might already use.