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rbp is used in SAVE_ARGS_IRQ to save the old stack pointer
in order to restore it later in ret_from_intr.

It is convenient because we save its value in the irq regs
and it's easily restored using the leave instruction.

However this is a kind of abuse of the frame pointer which
role is to help unwinding the kernel by chaining frames
together, each node following the return address to the
previous frame.

But although we are breaking the frame by changing the stack
pointer, there is no preceding return address before the new
frame. Hence using the frame pointer to link the two stacks
breaks the stack unwinders that find a random value instead of
a return address here.

There is no workaround that can work in every case. We are using
the fixup_bp_irq_link() function to dereference that abused frame
pointer in the case of non nesting interrupt (which means stack
changed).
But that doesn't fix the case of interrupts that don't change the
stack (but ...