RE: [Xen-ia64-devel] Why is cpl read as 0 in para-virtualization ?

["Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>]

> > > while I was modifying xen_early_setup, I tried to replace dcr
> > > kludge using
> > > cpl.  Unfortunatly, it doesn't work since cpl is read as 0
> > > (see vcpu.c).
> > > So my question is simple: why ?
> >
> > I think some places in Linux/ia64 determine whether code
> > is running in kernel or user mode by checking psr.cpl.
> Correct.
> One place is user_mode() is ptrace.h.
> Currently it is true if cpl != 0.  I have made a simple test: 
> I have changed 
> the code to cpl == 3, and cpl reads as 2.  Linux is booting 
> (dom0+domU).

IIRC, there was at least one place in assembly code which
checked psr.cpl==0 (maybe mca code?) but this was a longtime
ago (possibly 2.4.x) so the code may be gone.

There is Itanium architecture work going on to establish a
mechanism for determining whether an OS is running native or
as a (paravirtualized or fully-virtualized) guest.  I don't
know the status of this but am pretty sure it was checking
a currently reserved bit, possibly in cr.dcr.

Dan

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