Subject: Re: want to clarify powerpc assembly conventions in
	head.S	and	entry.S
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Chris Friesen <cfriesen@nortelnetworks.com>
Cc: linuxppc-dev list <linuxppc-dev@lists.linuxppc.org>,
       Linux Kernel list <linux-kernel@vger.kernel.org>
In-Reply-To: <407AA848.2000008@nortelnetworks.com>
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Date: Tue, 13 Apr 2004 08:54:19 +1000
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On Tue, 2004-04-13 at 00:31, Chris Friesen wrote:
> Benjamin Herrenschmidt wrote:
> >>You knew this was coming...  What's special about syscalls?  There's the
> >>r3 thing, but other than that...
> >
> > The whole codepath is a bit different, there's the syscall trace,
> > we can avoid saving much more registers are syscalls are function
> > calls and so can clobber the non volatiles, etc...
> 
> It appears that we always enter the kernel via "transfer_to_handler",
> and return via "ret_from_except".  Is this true? (I'm running on at
> least a 74xx chip.)

ret_from_syscall for syscalls, hash_page also has a different
return to userland path, and load_up_{fpu,altivec} have their own
retturn path.
On ppc32 currently, the entry is almost always the same except for
hash_page and load_up_{fpu,altivec}

> I want to insert two new bits of code, one that gets called before the
> exception handler when we drop from userspace to kernelspace, and one as
> late as possible before going back to userspace.  I need to catch
> syscalls, interrupts, exceptions, everything.
> 
> The entry one I planned on putting in "transfer_to_handler", just before
> "addi   r11,r1,STACK_FRAME_OVERHEAD".
> 
> I was planning on putting the exit one just after the "restore_user"
> label.  Will this catch all possible returns to userspace?

No.

Ben.


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