arch/tile: work around a hardware issue with the return-address stack

In certain circumstances we need to do a bunch of jump-and-link
instructions to fill the hardware return-address stack with nonzero values.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

3 files changed, 22 insertions, 2 deletions

diff --git a/arch/tile/include/asm/traps.h b/arch/tile/include/asm/traps.h
index 5f20f920f932..e28c3df4176a 100644
--- a/arch/tile/include/asm/traps.h
+++ b/arch/tile/include/asm/traps.h
@@ -64,7 +64,11 @@ void do_breakpoint(struct pt_regs *, int fault_num);
 
 
 #ifdef __tilegx__
+/* kernel/single_step.c */
 void gx_singlestep_handle(struct pt_regs *, int fault_num);
+
+/* kernel/intvec_64.S */
+void fill_ra_stack(void);
 #endif
 
-#endif /* _ASM_TILE_SYSCALLS_H */
+#endif /* _ASM_TILE_TRAPS_H */
diff --git a/arch/tile/kernel/intvec_64.S b/arch/tile/kernel/intvec_64.S
index 79c93e10ba27..2c181c864ef7 100644
--- a/arch/tile/kernel/intvec_64.S
+++ b/arch/tile/kernel/intvec_64.S
@@ -1156,6 +1156,18 @@ int_unalign:
         push_extra_callee_saves r0
         j       do_trap
 
+/* Fill the return address stack with nonzero entries. */
+STD_ENTRY(fill_ra_stack)
+        {
+         move   r0, lr
+         jal    1f
+        }
+1:      jal     2f
+2:      jal     3f
+3:      jal     4f
+4:      jrp     r0
+        STD_ENDPROC(fill_ra_stack)
+
 /* Include .intrpt1 array of interrupt vectors */
         .section ".intrpt1", "ax"
 
diff --git a/arch/tile/kernel/traps.c b/arch/tile/kernel/traps.c
index 2bb6602a1ee7..32acfd9e23d0 100644
--- a/arch/tile/kernel/traps.c
+++ b/arch/tile/kernel/traps.c
@@ -289,7 +289,10 @@ void __kprobes do_trap(struct pt_regs *regs, int fault_num,
                 address = regs->pc;
                 break;
 #ifdef __tilegx__
-        case INT_ILL_TRANS:
+        case INT_ILL_TRANS: {
+                /* Avoid a hardware erratum with the return address stack. */
+                fill_ra_stack();
+
                 signo = SIGSEGV;
                 code = SEGV_MAPERR;
                 if (reason & SPR_ILL_TRANS_REASON__I_STREAM_VA_RMASK)
@@ -297,6 +300,7 @@ void __kprobes do_trap(struct pt_regs *regs, int fault_num,
                 else
                         address = 0;  /* FIXME: GX: single-step for address */
                 break;
+        }
 #endif
         default:
                 panic("Unexpected do_trap interrupt number %d", fault_num);