On Apr 7, 2006, at 11:44 PM, Rahul Kulkarni wrote:

> Thanks Matt. appreciate your reply. If you can elaborate a bit more it
> would help
>
> I am running 1.6.1 on a powerpc 750..Haven't mucked around with the
> BAT stuff, and the USERSR. But it does appear like you pointed out
> that all of the physical memory is BAT mapped and the copyin/copy out
> leaves the user segment mapped in the USERSR. (Is this what you refer
> to as implementation? or is there something else)

The above is only true for the OEA PPCs.  Each processor/architecture
does something different but appropriate for themselves.

> I am concluding the above based on some experiments, From the kernel
> ddb i am able to look at the executable segment of a known userland
> process and comparing and verifying it with the object dump of  the
> executable which does indicate visibility and access..

Of course.  The kernel can do anything.  It's part of the TCB.

> A few more followup questions if you may:
>
> Though the kernel and user address spaces are completely disjoint,
> given the above facts couldn't a process running in the kernel /
> kernel itself trample and mess with the user space. ?

No.  User code does not run in kernel context.  Only the kernel does.
The portion of kernel that is performing actions for the user code
of process won't randomly trample parts of the user's address space.

Note that USERSR is only valid when the kernel is doing a copy{in,out}.
Also when the kernel returns to user processing, *all* SRs are replaced
with the user's SRs (including USERSR).  When the user process traps to
the kernel, USERSR is initialized to an inactive SR.

> Are you suggesting by controlling the USERSR in copyin/copy out the
> access to user space can be controlled in the kernel ?

of course.

> Is it default behaviour to leave the user segment mapped into the
> kernel's USERSR? Is it due to performace or what is the prime purpose?

See above. USERSR is only set to a valid value for small windows
in the kernel.

> Also any specifics why all the physical memory for powerpc's is BAT  
> mapped?

Because it allows easy and efficient access to physical memory.  It  
also limits
the amount of virtual memory needed by the kernel.