[uml-devel] (implementation idea) Re: uml and nonstandard memory splits?

[Blaisorblade <blaisorblade@yahoo.it>]

On Friday 24 March 2006 01:40, Jason Lunz wrote:
> On Thu, Mar 23, 2006 at 06:50:44PM -0500, Jeff Dike wrote:
> > You can make some progress by disabling CONFIG_MODE_TT - that will
> > cause UML to load at a normal address rather than the 0xa0000000 that
> > CONFIG_MODE_TT will force.
> >
> > That should get you to the point of trying to run init.
> >
> > Since you have skas3, my guess is that UML will be fine at that point.
> >
> > On a non-skas3 host, it will run in skas0 mode, and will expect to be
> > able to map a couple of pages at the top of the process address space
> > (hardcoded to 0xbfffe000 and 0xbffff000).  These offsets would need to
> > be calculated based on the host's split in order for skas0 to work.
>
> So I guess getting a 2.4 uml to work on any split other than 1G/3G is
> pretty unlikely. Oh well, I already knew 2.4 was on its way out. So this
> might be the thing that really forces me to use 2.6.

No, that's wrong - what David Lang described:
> I don't know about other non-standard memory configs, but there is an 
> explicit config option when compiling a uml kernel to support the 2G/2G 
> memory split.
was also supported by UML 2.4. I think unusual split and the UML trivial 
support for that were born and used when 4G/4G didn't exist yet (or maybe not 
even highmem?).

> 2.4 support aside, my aim is to have a single uml binary that will work
> on arbitrary host kernels. How difficult will it be to implement support
> for dynamic offsets for these pages in skas0?

I don't think it's easy - however, it can be done. Below my idea - Jeff, 
please comment on this

In assembly.
It goes like this:

call label /*this is coded like a relative jump, i.e. call +1*/
label: pop $eax /*we got the saved EIP*/

/* Now we can round EIP and subtract an offset to calculate the data page 
location.
*/

The only problem is that it must be done in assembly and at times this must be 
redone (we don't have enough registers to save the value across a 6-param 
syscall), and touching this code is a bit dangerous.

> Jason

-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade

	

	
		
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