Linux kernel ARM exception stack init

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tl;dr – We switch modes to supervisor and use that stack.

You are missing the key point of where control is handed to the CPU via the vector table and the mode is switched. See: entry-armv.S and __vectors_start. The vector stubs is the code where control is initially sent after the branch in the main vector table. The vector_stub macro saves three items; a corrected lr, r0 and the spsr of the excepted mode (as you noted).

The point you miss is, after this all exceptions switch to SVC_MODE and as such use the current tasks stack, which also has the thread_info structure. mode switching is a tough concept to get in ARM system level assembler. Registers that were previously set are now completely different. Pay attention to msr and cps type instructions. Things can change completely after them; I have been confused by this dozens of times.

The spsr is used as an index into a vector_stub table, which will normally jump to either __irq_svc or __irq_usr. Just scroll down to look at the bottom of the entry-arm.S which you already found.

Related: Physical address of ARM-Linux vector table

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