Re: arch/xen is a bad idea

[Andi Kleen <ak@suse.de>]

On Thu, Dec 16, 2004 at 01:19:06PM +0000, Alan Cox wrote:
> On Iau, 2004-12-16 at 14:09, Andi Kleen wrote:
> > Also e.g. for non performance critical 
> > things like changing MTRRs or debug registers it would be IMHO much 
> > cleaner to just emulate the instructions (the ISA is very well 
> > defined) and not change the kernel here.  From a look at Ian's list
> > the majority of the changes needed for Xen actually fall into
> > this category. 
> 
> There are so many problems in snooping and decoding instructions it
> isn't funny. Aside from the mmap pci buffer half way through instruction

Hmm? From what I remember reading the code of the Xen hypervisor
they are already emulating a lot of instructions (e.g. take a look
at xen/arch/x86/x86_32/seg_fixup.c) Emulating some more doesn't 
seem like a big issue to me.

> that will emulate type stuff there are a lot of awkward issues if you
> want to emulate multiple mtrr sets (you need PAT).

No way different from doing it with a magic interface. MTRR is exposed
to kernel subsystems (a lot of things in the service OS will want to
play with it) and to user space (with also a lot of users). The magic
interface will need to provide a full MTRR interface anyways.  The 
only difference would be basically how you tell the hypervisor to
change them.

[Not saying that PAT is actually needed for this, but as a general 
comment:]
Also using PAT for that would be definitely a good idea, MTRRs just
have so many problems that any step away from them is a good thing.


-Andi