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From: Martin Schaffner
Subject: Questions
Date: Mon, 27 Oct 2003 13:53:58 +0100 (MET)

I have two questions:

1) Performance:

In 06/04/2001, "sjames" wrote on slashdot

> The biggest problem for microkernels is that they have to switch
> contexts far more frequently than a monokernel (in general).
> For a simple example, a user app making a single system call. In a
> monokernel, The call is made. The process transitions from the user to
> the kernel ring (ring 3 to ring 0 for x86). The kernel copys any data
> and parameters (other than what would fit into registers) from
> userspace, handles the call, possably copies results back to
> userspace, and transitions back to ring3 (returns).
> In a microkernel, the app makes a call, switch to ring 0, copy data,
> change contexts, copy data to daemon, transition to ring3 in server
> daemon's context, server daemon handles call, transitions to ring 0,
> data copied to kernelspace, change contexts back to user process, copy
> results into user space, transition back to ring3 (return).

Am I right in thinking that a syscall on hurd/l4 would work differently?
If i understand correctly, it would work like this (example: reading data
from a file):

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), which in turn RPCs
the driver of the backing store (will probably reside in ring 0) for the data.

Assuming zero-copy, there is a minimum of data copying, but there are still
four context switches, which can't be done with super-fast IPCs, since they
concern three different processes (app, translator, driver).

Is it likely that l4/hurd will be slower than linux, for things like
filesystem operations?

2) Translators on linux

In 12/04/1999, Marcus Brinkmann wrote on help-hurd

> > make more useful translators. Then either Linux will switch to
> > translators or we will swich to not using Linux.
> > This is the greatest idea of Hurd, but not widely used yot.
> > And its already partially implemented in Linux
> I don`t think it is feasible to add this to Linux, especially if you
> consider security issues.

Does this mean that the Linux Userland Filesystem
(http://lufs.sourceforge.net/lufs/intro.html) poses security risks?


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