85xx Address space query

[Benjamin Herrenschmidt]

> > 1. User code executes ioctl
> > 2. interrupt goes to the kernel
> 
> On the interrupt the PR changes from 0 -> 1

The other way around actually :-)

> > 3. ioctl handler in driver gets invoked
> > The buffer pointer still contains 0x10000.
> >
> > How kernel code running in PR=0 accesses it and does the copy. I am  
> > not able to see a address space switch in the asm code of  
> > copy_tofrom_user.
> 
> There isn't a address space switch.  But address spaces exist at the  
> same time.  The user app is given 0..0xc000_0000 and the kernel uses  
> 0xc000_0000..0xffff_ffff.

You may want to add, to make things clearer, that the HW treats PID 0
specially. Translations in the TLB that have PID (we call it TID in the
TLB entry) 0 match any value of the PID register.

So kernel pages all have TID = 0, which means they are visible to all
processes, but have permissions set such that only the supervisor (ie.
PR = 0) can actually access them. MSR:PR is automatically switched to 0
by the processor when taking an interrupt, allowing the kernel thus to
access both its own pages and the pages of the current process.

Cheers,
Ben.