Re: [uml-devel] Explaination of system call function flow in TT mode

[Young Koh <young.koh@gmail.com>]

Hi, Thanks for the reply. Let me have one following question. 

As my understand, in SKAS mode, tracer and tracee are different host
processes, and they have UML kernel code and application code
respectively. (in TT mode, tracee has both kernel and application
code)

Suppose the tracee attmpts to invoke a host system call. then, it will
be intercepted by the tracer as in TT mode. (In TT mode, the tracer
turns off tracing and sends SIGUSR2 to the tracee, and then, the
signal handler in the tracee will execute the system call.) but, in
SKAS mode, because the tracee doesn't have the UML kernel code, only
the tracer knows about the system call code. The tracer cannot simply
send a signal to make the tracee execute the system call. Then how
does it do? Does the tracer execute the system call for the tracee?

Thank you,

> 
> In SKAS mode, instead, the UML scheduler switches back and forth between
> various "processes" (inside UML) which are all running inside userspace, and
> ptrace the various host threads (actually collapsed into one userspace
> thread, thanks to SKAS mode) which run only the real userspace code. So the
> ptraced context always executes userspace code and we never turn ptracing
> off.
> --
> Paolo Giarrusso, aka Blaisorblade
> Skype user "PaoloGiarrusso"
> Linux registered user n. 292729
> http://www.user-mode-linux.org/~blaisorblade
> 
>


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