Fix BUG_ON() reporting in real mode on powerpc

[Paul Mackerras]

On Wed, Feb 17, 2016 at 01:33:32PM +0530, Anshuman Khandual wrote:
> On 02/17/2016 12:46 PM, Balbir Singh wrote:
> >> > It might be a little better to do this:
> >> > 
> >> > 		bugaddr = regs->nip;
> >> > 		if (REGION_ID(bugaddr) == 0 && !(regs->msr & MSR_IR))
> >> > 			bugaddr += PAGE_OFFSET;
> >> > 
> >> > It is possible to execute from addresses with the 0xc000... on top in
> >> > real mode, because the CPU ignores the top 4 address bits in real
> >> > mode.
> > Good catch! Thank you
> > 
> > Changelog:
> >      Don't add PAGE_OFFSET blindly, check if REGION_ID is 0
> 
> Cant we use USER_REGION_ID directly ?

If we use USER_REGION_ID then the reader needs to know that the user
region is region 0 to understand the code.  Thus I think it is clearer
to use REGION_ID(bugaddr) == 0.  Whether or not the address is a user
region address is not really relevant to the question of whether it's
a physical address being accessed directly in real mode vs. a kernel
virtual address, which is what we're trying to determine.

Paul.