ARCv2: STAR 9000950267: Handle return from intr to Delay Slot #2

Returning to delay slot, riding an interrupti, had one loose end.
AUX_USER_SP used for restoring user mode SP upon RTIE was not being
setup from orig task's saved value, causing task to use wrong SP,
leading to ProtV errors.

The reason being:
 - INTERRUPT_EPILOGUE returns to a kernel trampoline, thus not expected to restore it
 - EXCEPTION_EPILOGUE is not used at all

Fix that by restoring AUX_USER_SP explicitly in the trampoline.

This was broken in the original workaround, but the error scenarios got
reduced considerably since v3.14 due to following:

 1. The Linuxthreads.old based userspace at the time caused many more
    exceptions in delay slot than the current NPTL based one.
    Infact with current userspace the error doesn't happen at all.

 2. Return from interrupt (delay slot or otherwise) doesn't get exercised much
    after commit 4de0e528 ("Really Re-enable interrupts to avoid deadlocks")
    since IRQ_ACTIVE.active being clear means most returns are as if from pure
    kernel (even for active interrupts)

Infact the issue only happened in an experimental branch where I was tinkering with
reverted 4de0e528

Cc: stable@kernel.org # v4.2+
Fixes: 4255b07f ("ARCv2: STAR 9000793984: Handle return from intr to Delay Slot")
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/kernel/entry-arcv2.S

@@ -211,7 +211,11 @@ debug_marker_syscall:
 ; (since IRQ NOT allowed in DS in ARCv2, this can only happen if orig
 ; entry was via Exception in DS which got preempted in kernel).
 ;
-; IRQ RTIE won't reliably restore DE bit and/or BTA, needs handling
+; IRQ RTIE won't reliably restore DE bit and/or BTA, needs workaround
+;
+; Solution is return from Intr w/o any delay slot quirks into a kernel trampoline
+; and from pure kernel mode return to delay slot which handles DS bit/BTA correctly
+
 .Lintr_ret_to_delay_slot:
 debug_marker_ds:
 
@@ -222,18 +226,23 @@ debug_marker_ds:
 	ld	r2, [sp, PT_ret]
 	ld	r3, [sp, PT_status32]
 
+	; STAT32 for Int return created from scratch
+	; (No delay dlot, disable Further intr in trampoline)
+
 	bic  	r0, r3, STATUS_U_MASK|STATUS_DE_MASK|STATUS_IE_MASK|STATUS_L_MASK
 	st	r0, [sp, PT_status32]
 
 	mov	r1, .Lintr_ret_to_delay_slot_2
 	st	r1, [sp, PT_ret]
 
+	; Orig exception PC/STAT32 safekept @orig_r0 and @event stack slots
 	st	r2, [sp, 0]
 	st	r3, [sp, 4]
 
 	b	.Lisr_ret_fast_path
 
 .Lintr_ret_to_delay_slot_2:
+	; Trampoline to restore orig exception PC/STAT32/BTA/AUX_USER_SP
 	sub	sp, sp, SZ_PT_REGS
 	st	r9, [sp, -4]
 
@@ -243,11 +252,19 @@ debug_marker_ds:
 	ld	r9, [sp, 4]
 	sr	r9, [erstatus]
 
+	; restore AUX_USER_SP if returning to U mode
+	bbit0	r9, STATUS_U_BIT, 1f
+	ld	r9, [sp, PT_sp]
+	sr	r9, [AUX_USER_SP]
+
+1:
 	ld	r9, [sp, 8]
 	sr	r9, [erbta]
 
 	ld	r9, [sp, -4]
 	add	sp, sp, SZ_PT_REGS
+
+	; return from pure kernel mode to delay slot
 	rtie
 
 END(ret_from_exception)