On Fri, Feb 24, 2012 at 10:12:06PM +0000, Catalin Marinas wrote:

Hi Uwe,

2012/2/24 Uwe Kleine-K?nig [off-list ref]:

quoted

On Thu, Feb 16, 2012 at 10:20:02PM +0000, Russell King - ARM Linux wrote:

quoted

On Thu, Feb 16, 2012 at 09:18:10PM +0100, Uwe Kleine-K?nig wrote:

quoted

+ ? .macro ?v7m_exception_entry
+ ? cpsid ? i
+ ? tst ? ? lr, #0x8 ? ? ? ? ? ? ? ?@ check the return stack
+ ? bne ? ? 1f ? ? ? ? ? ? ? ? ? ? ?@ exception on process stack
+ ? add ? ? r12, sp, #32 ? ? ? ? ? ?@ MSP before exception
+ ? stmdb ? sp!, {r4-r12, lr} ? ? ? @ push unsaved registers
+ ? b ? ? ? 2f
+1:
+ ? mrs ? ? r12, psp ? ? ? ? ? ? ? ?@ get the process stack
+ ? sub ? ? sp, #S_FRAME_SIZE
+ ? stmia ? sp, {r4-r12, lr} ? ? ? ?@ push unsaved registers
+ ? ldmia ? r12, {r0-r3, r6, r8-r10} @ load automatically saved registers
+ ? add ? ? r12, sp, #S_R0
+ ? stmia ? r12, {r0-r3, r6, r8-r10} @ fill in the rest of struct pt_regs

I guess this means that pt_regs no longer contains r0..pc, cpsr, old_r0
on this Cortex-M ?

Just to reply to Russell - yes, this part needs changing (could be the
pt_regs saving or just the signal and trace code that copies them to
user so that we preserve the ABI).

quoted

I stared at the code now for some time and I wonder if it wouldn't be
the most nice solution to just do something like this on exception
entry:

? ? ? ?cpsid ? i
? ? ? ?sub ? ? sp, #S_FRAME_SIZE
? ? ? ?stmia ? sp, {r0-r12}
? ? ? ?put_the_right_sp_to_sp[13]
? ? ? ?put_lr_returnaddr_and_xPSR_from_right_stack_to_sp[14-16]

For returning you could just do:

? ? ? ?add ? ? sp, #S_FRAME_SIZE
? ? ? ?cpsie ? i
? ? ? ?bx ? ? ?lr

For this kind of returning, do we actually change between Process and
Handler stack?

According to ARMARM-v7-M when pc is written to 0xfXXXXXXX (with X
denoting don't care) also the stack is restored.

But we still need to set the pt_regs to the user stack. The kernel
would touch all of them if a signal is to be delivered (actually
restoring them when returning from the signal handler).

Ah, I thought only r0 might be changed. I will bear that in mind.

Thanks for your insights
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-K?nig            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |