Re: CONFIG_FRAME_POINTER and module vermagic

["Christopher Friesen" <cfriesen@nortel.com>]

A while back there was a post that CONFIG_FRAME_POINTER doesn't affect 
calling conventions and doesn't need to be in vermagic.

One of my coworkers seems to think otherwise, and I don't know enough 
about the issue to know for sure.  Could someone with i386 knowledge 
comment on his thoughts?

Here's what he's currently thinking:

1) regs->ebp hold a copy of the stack frame pointer. It's value is 
conserved through any function that are compiled with FRAME_POINTER on.

2) (unsigned long *)(regs->ebp + 4) is the "pc" of the caller (like the 
link register on PPC which is relative to "fp")

3) The profile_pc function usually look directly at "pc" to do it's 
profiling magic but sometimes (when the current "pc" is inside a 
lock_function, we're SMP, and CONFIG_FRAME_POINTER is enabled) it uses 
"regs->ebp+4" to be more accurate on the profiling. In other word 
profile_pc doesn't want to create a profiling entry that would show 
redundant information about being stuck into a spin_lock...

So, if the kernel was built with SMP and FRAME_POINTER, a module wasn't, 
the module used ebp as a general register, then blocked in a spinlock 
when profile_pc started...then a regs->ebp value of something 
interesting (like "0", for instance) could cause interesting behaviour.

Seems reasonable to me, but like I said, I'm not an expert on i386.

Chris
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