Re: [uml-devel] UML hangs with hrtimer test module

[Richard Weinberger <richard@nod.at>]

Am Donnerstag, 29. März 2018, 22:20:47 CEST schrieb Joel Fernandes:
> Thanks a lot! I am wondering why the same compiler works when running
> the test for a regular image. Maybe different compiler flags. Anyway
> good to learn this.
> 
> Also one more slightly OT question, why is UML only doing UP ? Is it
> extremely hard to do SMP for UML?

Long story short, nobody implemented SMP so far. :-)
Because SKAS3/0 we had a SMP implementation of TT mode.

In terms of UML implementing SMP means having multiple threads
that handle the userspace loop in arch/um/os-Linux/skas/process.c.

We could also do a poor man's SMP implementation first, where only user 
processes run in parallel.
IOW userspace() in arch/um/os-Linux/skas/process.c is still a single thread 
but it let's run up to N user space thread and only if the call into the 
kernel we degrade to UP.

Adding SMP is not extremely hard but it requires a lot of re-work of the UML 
core and introduces tons of new issues.

That said, volunteers are welcome!

Thanks,
//richard