Re: context switch vs. signal delivery [was: Re: Accelerating user mode

From: Richard Zidlicky (
Date: Mon Aug 05 2002 - 15:44:15 EST

On Mon, Aug 05, 2002 at 03:46:07PM +0200, Udo A. Steinberg wrote:
> On Sat, 03 Aug 2002 10:29:42 -0500
> Jeff Dike <> wrote:
> > said:
> > > the alternatives like a seperate process and ptrace are not pretty either
> I have implemented a usermode version of the Fiasco -kernel that uses
> a seperate process for the kernel and one process for each task. The kernel
> process attaches to all tasks via ptrace.
> When the kernel wants to change the MM of a task it puts some trampoline code
> on a page mapped into each task's address space and has the task execute that
> code on behalf of the kernel.
> With that setup we have complete address space protection without all the
> trouble of jail at the expense of a few context switches for each mmap, munmap
> or mprotect operation.

very interesting, what is the handiest way to do "syscalls" in this model?
Ptrace is still basically signal driven so I would expect it has still some
unnecessary overhead?

> I would also very much like an extension that would allow one process to modify
> the MM of another, possibly via an extended ptrace interface or a new syscall.
> Also it would be nice if there was an alternate way to get at the cr2 register,
> trap number and error code other than from a SIGSEGV handler.

that's what signals are for, too bad they are slow.

> > Then, the current UML tracing thread would handle the kernel side of things
> > and sit in its own address space nicely protected from its processes.
> Yes. I already have this part working for our kernel, so it's not just theory.
> I believe things could run yet another bit faster if we didn't have to do the
> trampoline map operations.

they are very expensive because of the way ptrace accesses the other process
memory, did you try a piece of shared memory ?

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