Re: [Xen-devel] Xen as a kernel module

[Jacob Gorm Hansen <jacobg@xxxxxxx>]

Tobias Hunger wrote:
> > Right now Xen is mapped somewhere in top of memory, I am not sure how
> > domains are kept out of there, but I suppose it has to do with segments.
>
> As I understand this xen runs in ring0 and pushes the guests kernels one ring 
> up into ring1 and then uses traps to allow the guest OSes to trap into the 
> hypervisor as necessary.
>
> > The good thing about that is that hypercalls are cheap, and in Xen1.x
> > I/O was cheap as well.
>
> Cheap where? In dom0 or the VMs?
Both. Assuming that context switching is an expensive operation, and 
that most of the cost comes from flushing the TLBs, the current model 
has more overhead than a model where the drivers are mapped into the 
same address space (though still protected from domU access other that 
through hypercalls) as the currently running domain. Xen already 
contains magic that allows it to be permanently mapped into running 
domains, while still being protected from accidential/malicious access.
I am calling that monolithic, because that is what standard linux does, 
and what the old Xen 1.x did. The current model is more like a 
microkernel, such as L4 or Mach (though of course a lot more clever, no 
offense people ;-))
Apart from perhaps increasing performance, I was saying it would be cool 
if the Xen-interface could be provided on platforms such as Windows or 
native Linux. People have done similar stuff before (e.g. VMWare vmmon, 
the Adeos nanokernel, coLinux), so it should be possible. It seems Xen 
would have better chances of world domination that way, if that is a 
goal. It would also save some duplication of effort in porting guestOSes 
to the different models out there.
Anyway, it was just an idea, and it is likely that with future hardware 
the Xen NGIO/microkernel model will finally be overhead-free, vendors 
will start writing Xen-VM drivers, and we will all live happily ever 
after :-).
Jacob


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