Query - backup of User Mode regs on segfault

Hi,
I'm studying the code of how the kernel handles a User Space segfault.
Couple of questions on that:

1. What is the 1st kernel instruction that gets executed when the hardware
detects a segfault? Is it __dabt_usr: in arch/arm/kernel/entry-armv.S  OR
does s'thing happen before that also?

2. Are the User Space regs always backed up on top of the stack OR is the
location arbitrary on the stack?

3. Is the member thread_info->cpu_context used for User Space regs backup
anytime?

4. In pt_regs what is the ARM_ORIG_R0 member?

TIA,
Indro
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