Re: Creating tasks and the l4_task_map function

On Friday, 22 April 2022 01:16:44 CEST Paul Boddie wrote:
> 
> On Thursday, 21 April 2022 22:57:40 CEST Adam Lackorzynski wrote:
> > 
> > Maybe it helps to share some code here?
> 
> I suppose it might be best to condense this into an example that uses the
> basic L4Re APIs instead of my own libraries (that wrap those APIs), so as
> not to confuse things. I'll try and put that together tomorrow.

So, I finally got to looking at this and making it self-contained, with an 
archive of the code available here:

https://www.boddie.org.uk/downloads/tests_l4re.tar.bz2

This is a package that should work within the L4Re build system containing two 
source files: a trivial program (exec_payload_l4re.c) and a loader program 
that attempts to create a new task to load and start the trivial program 
(exec_l4re.cc). Both programs are statically linked.

Note that the loader program does not do all the work to start the program, 
focusing only on being able to resolve the initial page fault. So, although it 
attempts to set up various capabilities and other resources, it doesn't even 
initialise the stack or map in other regions that would be needed to actually 
run the program.

The log when running the loader program using the supplied configuration will 
report page faults occurring as follows:

exec_l4r| page_fault(0x1000ae4, 0x1000ae3) -> 0x1000ae0 (0x4)...
exec_l4r| -> l4_fpage(0x40000, 18, 0x5) @ 1000000
exec_l4r| page_fault(0x1000ae5, 0x1000ae3) -> 0x1000ae0 (0x5)...
exec_l4r| -> l4_fpage(0x40000, 18, 0x5) @ 1000000

Here, the received fault is decoded and the flexpage for returning is 
described. When I switch on page fault and IPC logging (P*, I*, IR+) in jdb, I 
get entries like this:

     0047 answ [00000040] L=0 err=0 (OK) (1000038,40495)                                                                                                
pf:  0043 pfa=0000000001000ae3 ip=0000000001000ae3 (rp) spc=0xffffffff13dc5cb8
          err=15
ipc: 0047 send rcap->[C:INV] DID=43 L=0 [0000000000000040]
          (0000000001000038,0000000000040495) TO=INF

Here, thread 0047 belongs to the loader program, and 0043 belongs to the 
trivial payload program.

It is entirely possible that I am not sufficiently initialising the created 
task, but it surprises me that the page fault does not get resolved, and I did 
some old-fashioned debugging with Fiasco to establish that the mapping does 
occur. One difference between the above and my previous report is that the 
mapped region is now significantly smaller, since I am now using a binary file 
installed in the "rom" as a module, not the initially generated ELF binary.

When looking for other code that might be doing this kind of thing, I 
remembered L4Linux which seems to use the C APIs. Much of what it is doing is 
similar to what I have been trying, although it seems to use the "alien" 
thread mechanism at various points, which I don't fully understand.

Anyway, I hope that I am making some obvious mistake that just hasn't been 
obvious to me. Although there were examples in the package that was available 
via Subversion (and maybe still is available via Git), they covered alien 
threads and the vCPU support, but I couldn't find anything related to task 
creation in its own right. Interestingly, I did find this:

https://github.com/phipse/L4RTEMS/blob/master/l4/pkg/RTEMS_wrapper/server/src/
wrapper_1.cc

But it is also using the vCPU feature. However, it does seem to attempt ELF 
header decoding and related activities, so it might still prove to be a useful 
reference.

Thanks once again for listening!

Paul