Hi David,
On 2025-09-22 05:37, [email protected] wrote:
Hello L4 hackers, I hope you are all doing well today.
I am hoping to discuss the topic of threads to better my understanding
of it, as it seems to be a major building block in L4Re. I have the
following questions (using the c++ based libraries.):
1. Is Pthread the official way to create threads in an L4Re
application?
That is at least the high level primitive that you should use if you
have no good reason to use the low level L4 primitives and want to use
any of the POSIX / libc functionality inside your thread.
2. I have made a L4 thread (to understand how threads work in L4)
without the help of pthreads by setting up the Attr, which involved in
setting the exception handlers(using rm()), pager (using rm()),
instruction pointers, stack pointers, binding, ex_reg() and finally
running. All seems well until I try printing a message.
It seems seems I am unable to use the printf() function in the
instruction pointer function of said thread. The error I get is :
ttest | l4re_itas[rm]: unhandled read page fault at 0x141
pc=0x100f8a0
ttest | l4re_itas: rom/thread_test: Unhandled exception:
PC=0x100f8a0 PFA=0x140 LdrFlgs=0x0
However, if I use printf in a pthread, none of these errors pop up.
2a. Am I using the wrong exception handler and pager (I got both
from rm)?
2b. Also, the "l4_vcon_send(L4_BASE_LOG_CAP,..,...) function seems
to work fine in either case, shouldn't printf() eventually call this
function as
after a series of indirections?
2c. What exc_handler and pager do pthreads use? Why are they able
to handle the pager faults and exceptions from printf? Can I
appropriate
them for the L4 threads I started?
What you are seeing is a side-effect of libpthread being quite tightly
integrated with the libc. While it is true that printf will call
l4_vcon_send somewhen down the line it will use some (potentially
thread-local) internal buffers to print its output. While you can mix
libpthread threads with native L4 threads you must take care to call no
function from the libc that would make any use of TLS storage, unless
you set this up yourself.
If you want to write normal applications std::thread and pthread threads
are the way to go.
3. It seems the Virtual Machines (VMs) that are on the hypervisor are
also closely related with threads. I assume these are Vcpu threads and
not pthreads.
3a. Am I correct in this assumption?
3b. How are the exception handlers and pagers set up for these
Vcpus?
Yes, a vCPU is in the end only a thread with some extended state. A
thread in vcpu mode provides an additional exception execution context.
When a trap from avcpu occurs the kernel will switch the vCPU to the
exception entry point. See entry_ip and entry_sp here
https://l4re.org/doc/structl4__vcpu__state__t.html#a1f45174954aa47bb2f21b7b90bed250e
These must be set as part of the vcpu state before starting the vcpu.
The exception handler must take care of handling page-faults generated
by threads user code.
I hope this helps :)
Best regards,
- Marcus
Thank you so much for your time,
David
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