[RFC PATCH v2 2/6] powerpc: Exception hooks for context tracking subsystem

[Li Zhong]

On Fri, 2013-04-05 at 13:50 +1100, Paul Mackerras wrote:
> On Fri, Mar 29, 2013 at 06:00:17PM +0800, Li Zhong wrote:
> > This is the exception hooks for context tracking subsystem, including
> > data access, program check, single step, instruction breakpoint, machine check,
> > alignment, fp unavailable, altivec assist, unknown exception, whose handlers
> > might use RCU.
> > 
> > This patch corresponds to
> > [PATCH] x86: Exception hooks for userspace RCU extended QS
> >   commit 6ba3c97a38803883c2eee489505796cb0a727122
> > 
> > Signed-off-by: Li Zhong <zhong at linux.vnet.ibm.com>


Hi Paul, 

Thanks for your review! Please check my answers below, and correct me if
any errors.

> Is there a reason why you didn't put the exception_exit() call in
> ret_from_except_lite in entry_64.S, and the exception_entry() call in
> EXCEPTION_PROLOG_COMMON?  That would seem to catch all these cases in
> a more centralized place.

It seems to me that ret_from_except_lite and EXCEPTION_PROLOG_COMMON are
also used by interrupts, where I think we don't need the hooks. So using
this way could help to avoid adding overhead to these code path
(interrupts, and some exit path of syscall). 

And I think adding the hook on higher level code seems a little easier
for reading and checking. It seems that some exceptions don't use
EXCEPTION_PROLOG_COMMON, and some don't go ret_from_except_lite exit
path (like fp unavailable might go directly to fast_exception_return ).
Maybe fast_exception_return is a centralized place for us to return to
user space? But it still adds some overheads which is not necessarily
needed. 

And I think it also makes the implementation here consistent with the
style that x86 uses. 

> Also, I notice that with the exception_exit calls where they are, we
> can still deliver signals (thus possibly taking a page fault) or call
> schedule() for preemption after the exception_exit() call.  Is that
> OK, or is it a potential problem?

If I understand correctly, I guess you are talking about the cases where
we might return to user space without context state correctly being set
as in user?

There is user_enter() called in do_notify_resume() in patch #3, so after
handling the signals we always call user_enter(). 

There are also some changes of the context_tracking code from Frederic,
which might be related: ( they are now in tip tree, and url of the
patches for your convenience https://lkml.org/lkml/2013/3/1/266 )

6c1e0256fad84a843d915414e4b5973b7443d48d
context_tracking: Restore correct previous context state on exception
exit. 

With this patch, if a later exception happened after user_enter(),
before the CPU actually returns to user space, the correct context
state(in user) is saved and restored when handling the later exception. 

Patch #6 converts the code to use these new APIs, which is currently not
available in powerpc tree. 

b22366cd54c6fe05db426f20adb10f461c19ec06
context_tracking: Restore preempted context state after
preempt_schedule_irq

With this patch, the user context state could be correctly restored
after schedule returns. 

Thanks, Zhong

> Paul.
>