exception -> l4_ipc ?

Adam Lackorzynski adam at os.inf.tu-dresden.de
Mon Jun 13 22:04:45 CEST 2005

On Mon Jun 13, 2005 at 15:03:35 -0400, Andrew Davenport wrote:
> i see thread::raise_exception(ts), and i know this is where l4 notices a linux
> sys call (int 0x80), however, i do *not* see where the ipc is sent to the
> linux server to tell it that a syscall has occured (and to get the parameters
> and handle it)...

As already mentioned, there are two ways an exception gets signalled to
user land. One is the exception IPC mechanism, the other is the "IDT"

Both mechanisms start in Thread::handle_slow_trap().

Thread::raise_exception() is used for the idt mechanism. For this to
work, a thread needs to register exception handlers (basically function
pointers in a table for each exception) that are called if an exception
is raised. So if the thread executes an 'int $0x80' it traps into the
kernel and when resuming to user land  continues its execution at the
code defined for exception 13 (int 80 will trigger an exception 13). All
thread state is preserved. No IPC is involved.

The other mechanism, exception IPC, basically works like a page fault.
When an exception happens, the kernel generates an exception to the
exception handler along with the state and waits for a response to set
some new state for the thread which then continues. In handle_slow_trap
this is initiated in the Thread::snd_exception() function, and goes on
with Thread::exception() where the IPC is sent and the response

Adam                 adam at os.inf.tu-dresden.de
  Lackorzynski         http://os.inf.tu-dresden.de/~adam/

More information about the l4-hackers mailing list