Re: [uml-devel] System Call Mechanism

[BlaisorBlade <blaisorblade_spam@yahoo.it>]

Alle 22:30, giovedì 19 febbraio 2004, kuas ha scritto:
> Hello,
>
> I'm posting this question to here since this is development problem, the
> other newsgroup seems to be more like users problem and I would think
> this forum would be able to explain more about my question. I will post
> to the other newsgroup if that's the  one where I should really be posting

Perfect choice. I'll try to answer, for things I know or I *think* to know.

> I seems to me the new or current implementation of system call involves
> conversion from interupt 80 at the host OS into signals back in UML
> kernel. Is this the same mechanism for the current TT and SKAS? Or  TT
> mode still using ptrace thread tracing mechanism to intercept the system
> call?

SKAS mode, the more modern one, still uses the same interception mechanism as 
TT: i.e. ptraces the child, intercepts the syscall, nullifies it (i.e. the 
host kernel receives a getpid syscall) and goes on. Look at these files:

arch/um/kernel/skas/process.c: handle_trap () and userspace().
arch/um/kernel/skas/syscall_*.c

Only difference is that in SKAS mode you have one single child process being 
ptraced, on the host; it runs the virtual processes code, and does syscall 
with int 0x80 which are intercepted. To do a context switch, UML uses setjmp 
and longjmp to switch the stack state and PTRACE_SWITCH_MM (implemented on 
the host inside arch/i386/kernel/ptrace.c, read the skas patch) to switch the 
virtual mappings of the child at once. More details about memory handling at 
request, since you asked for one exact thing and maybe are not interested in 
this.

> If there is a signal from the host to UML kernel process, there
> must be a signal handler registered by the UML kernel.
>
> Any quick deeper overview about this mechanism would be really welcome.
> Thanks in advance.


-- 
Paolo Giarrusso, aka Blaisorblade
Linux registered user n. 292729



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