Re: vfork on ARM

[Patrick Oppenlander <pattyo.lists@gmail.com>]

On 01/04/16 12:53, Rich Felker wrote:
> On Fri, Apr 01, 2016 at 11:42:47AM +1100, Patrick Oppenlander wrote:
>> I'm looking at what would be involved in using musl on a nommu arm system.
>>
>> As far as I know SYS_vfork is available on ARM, but musl is
>> currently falling back to fork.
>>
>> Are there any plans to support vfork on ARM and other architectures?
> It's trivial to add vfork, but the usefulness is limited without other
> changes:

vfork should also be more efficient than fork which may be motivation 
for supporting it as an optimisation on mmu targets.

It's also theoretically possible to run a nommu kernel on an mmu capable 
target.

> 1. To my knowledge, all nommu ARM systems are thumb[2]-only, so
> supporting them as targets requires adapting all the asm files to
> support building as thumb. This is a task in progress and, as long as
> we only care about thumb2 (available on armv7-m, i.e. Corext-M3 and
> up, I think) it's almost done.

OK that's great!

> 2. For pre-v7, there's no way to do atomics without kernel help, and
> no established kernel API for this as far as I know. For v7-m this is
> probably not a problem.

V6K has support for hardware atomics too.

v7-m supports 32-bit atomics but drops support for 64-bit (no LDREXD or 
STREXD). Is a problem for musl?

Do you know if v7-m has the hardware TLS registers?

> 3. Running on nommu without shareable program text is not much fun;
> execve is really slow (memcpy of full program) and you need lots of
> memory. Some people at ST have implemented an FDPIC abi for ARM which
> solves this problem, but it's not upstream in the toolchain or kernel,
> and the relocation types it needs are not officially assigned. Getting
> it officially stabilized, supported, and forward-ported to modern tool
> versions is going to be a lot of work. Here are some slides on it:
>
> http://www.slideshare.net/linaroorg/sfo15406-arm-fdpic-toolset-kernel-libraries-for-cortexm-cortexr-mmuless-cores

Thanks for the link. I wasn't aware of this.

> Without FDPIC, it's possible to build a toolchain that produces
> static-PIE executables that will work on nommu (with my recently
> committed kernel patch for running non-FDPIC PIE ELF files on nommu,
> and some additional work still needed to hook it up to work on ARM)
> but these cannot use a shared mapping of the program.
>
> If you or anyone else is up for helping with these tasks that would be
> great.

Right now I'm working on my own small kernel which will (hopefully) 
implement enough of the linux syscall interface to be useful. It's meant 
for small embedded microcontrollers where 4MiB of RAM is considered 
luxurious.

It's based on the now abandoned Prex operating system 
(http://prex.sourceforge.net/) but is a major fork which goes back to a 
traditional monolithic kernel model. I've replaced the C libary with 
musl and userspace is currently toybox.

I'm planning on releasing on github (BSD or no-license) once I can boot 
the first targets (arm-mmu and arm-nommu) to a working userspace and 
pass some unit tests.

Maybe once I've learnt enough about how all this stuff works I'll be 
able to contribute to other projects like linux/musl.

         Patrick