Re: [PATCH 2 of 3] [KVM] Add DCR access information to struct kvm_run

[Hollis Blanchard <hollisb@us.ibm.com>]

On Monday 07 April 2008 22:54:41 David Gibson wrote:
> On Mon, Apr 07, 2008 at 10:25:32PM -0500, Hollis Blanchard wrote:
> > On Monday 07 April 2008 20:11:28 David Gibson wrote:
> > > On Mon, Apr 07, 2008 at 03:53:33PM -0500, Hollis Blanchard wrote:
> > > > 1 file changed, 7 insertions(+)
> > > > include/linux/kvm.h |    7 +++++++
> > > >
> > > >
> > > > Device Control Registers are essentially another address space found
> > > > on PowerPC 4xx processors, analogous to PIO on x86. DCRs are always
> > > > 32 bits, and are identified by a 32-bit number.
> > >
> > > Well... 10-bit, actually.
> >
> > The mtdcrux description in the ppc440x6 user manual says the following:
> >
> > 	Let the contents of register RA denote a Device Control Register.
> > 	The contents of GPR[RS] are placed into the designated Device Control
> > Register.
> >
> > I take that to mean that we must worry about 32 bits worth of DCR
> > numbers. Perhaps I should say "no more than" rather than "always".
>
> I think that's less misleading.  mtdcrux is very new, anything which
> only has the mtdcr instruction certainly can't take DCR numbers above
> 10 bits, and I would expect that even on chips with mtdcrux the DCR
> bus is probably still only 10-bits, although it could be extended.

We're defining a kernel/userspace interface here, and since the hardware is 
capable of 32-bit DCR numbers, I don't think it makes any sense to not 
support that. Also, we would just end up placing that number into a u32 
anyways, so... :)

-- 
Hollis Blanchard
IBM Linux Technology Center