arm64: reduce stack use in irq_handler

The code for switching to irq_stack stores three pieces of information on
the stack, fp+lr, as a fake stack frame (that lets us walk back onto the
interrupted tasks stack frame), and the address of the struct pt_regs that
contains the register values from kernel entry. (which dump_backtrace()
will print in any stack trace).

To reduce this, we store fp, and the pointer to the struct pt_regs.
unwind_frame() can recognise this as the irq_stack dummy frame, (as it only
appears at the top of the irq_stack), and use the struct pt_regs values
to find the missing interrupted link-register.

Suggested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 7 insertions, 5 deletions

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 2284c296e3f7..0667fb7d8bb1 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -178,7 +178,7 @@ alternative_endif
         mrs     \rd, sp_el0
         .endm
 
-        .macro  irq_stack_entry, dummy_lr
+        .macro  irq_stack_entry
         mov     x19, sp                 // preserve the original sp
 
         this_cpu_ptr irq_stack, x25, x26
@@ -196,10 +196,12 @@ alternative_endif
         add     x26, x25, x26
         mov     sp, x26
 
-        /* Add a dummy stack frame */
-        stp     x29, \dummy_lr, [sp, #-16]!           // dummy stack frame
+        /*
+         * Add a dummy stack frame, this non-standard format is fixed up
+         * by unwind_frame()
+         */
+        stp     x29, x19, [sp, #-16]!
         mov     x29, sp
-        stp     x19, xzr, [sp, #-16]!
 
 9998:
         .endm
@@ -229,7 +231,7 @@ tsk	.req	x28		// current thread_info
         .macro  irq_handler
         ldr_l   x1, handle_arch_irq
         mov     x0, sp
-        irq_stack_entry x22
+        irq_stack_entry
         blr     x1
         irq_stack_exit
         .endm