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RE: Questions

From: Volkmar Uhlig
Subject: RE: Questions
Date: Mon, 27 Oct 2003 08:44:02 -0800

> -----Original Message-----
> From: Marcus Brinkmann
> Sent: Monday, October 27, 2003 6:05 AM
> To: Martin Schaffner
> Cc: address@hidden
> Subject: Re: Questions
> > The app has previously aquired a capability for the file it 
> > wants to read, and allocated a buffer. When it calls "read",
> > the glibc function makes an RPC directly to the filesystem 
> > translator (no going to ring 0, assuming the translator is 
> > owned by the same user as the calling process),
> No, no.  For any IPC you make a system call to the kernel.  
> Even for local IPC within a process, and definitely between 
> processes.

In the case of intra-process communication L4 X.2 does not necessarily
enter privileged mode.  If the destination thread is blocked and the
sender performs a blocking call the IPC will be executed completely in
user land without the overhead of a privilege-level switch.  The notion
of a "system call" does not necessarily imply a level switch, but is
rather an abstraction for a kernel-provided interface.  The same is true
for certain ex-regs scenarios.

- Volkmar

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