Re: [uml-devel] Using SKAS, any examples?

[Blaisorblade <blaisorblade@yahoo.it>]

On Sunday 08 May 2005 20:09, Ian Rogers wrote:
> Blaisorblade wrote:

> Thanks. I will implment an emulator through this mechanism. My current
> mechanism performs:
>
> loadByte(addr) {
>  pte = addr >> 12;
>  offset = addr & 0xFFF;
>  result = memory[ptr][offset]
> }
Ok, memory is a square array.
> you can imagine the store... mechanisms. Obviously the cost of the
> ptrace calls is going to be a slow down compared to this.
Well, if it's done only at setup, ok. However, can you please explain to me 
why it isn't possible to do it without SKAS?
> I hope to look into creating a patch to allow multiple segments in a
> linux process. So an extension to skas on i386 and I can get my %fs wish
> :-)
Well, it should be already doable in SKAS, with PTRACE_LDT (which relates to 
modify_ldt() the same way that writing DO_MMAP relates to mmap()). Then, 
after *setting* an appropriate descriptor in %fs (with PTRACE_POKEUSR if you 
need), you're done.

> btw: a similar thing should be possible on PowerPC given that
> segmentation can be disabled for either or both the instruction and data
> cache. It would be neat to wrap this up in a module for the performance
> improvement of emulators.

> Glad it was useful and thanks for your help - it's taught me what to do
> for my emulator code. I hope I can post some more things back in the
> future. Would the multi-segment stuff be useful for more people? Is
> there an obvious reason to avoid multi-segments?

Well, I've proposed some time ago to use segmentation for UML, but the 
proposal was dropped "because an access through segmentation is a little 
slower".

Quite frankly, I'm quite unconvinced (because any access passes through 
segmentation, even if normally segments have a 0 base and a disabled limit); 
beyond, Xen uses segmentation in the same way that I was proposing for UML, 
IIRC.
-- 
Paolo Giarrusso, aka Blaisorblade
Skype user "PaoloGiarrusso"
Linux registered user n. 292729
http://www.user-mode-linux.org/~blaisorblade




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