ARM: Fix undefined instruction exception handling

While trying to get a v3.5 kernel booted on the cubox, I noticed that
VFP does not work correctly with VFP bounce handling.  This is because
of the confusion over 16-bit vs 32-bit instructions, and where PC is
supposed to point to.

The rule is that FP handlers are entered with regs->ARM_pc pointing at
the _next_ instruction to be executed.  However, if the exception is
not handled, regs->ARM_pc points at the faulting instruction.

This is easy for ARM mode, because we know that the next instruction and
previous instructions are separated by four bytes.  This is not true of
Thumb2 though.

Since all FP instructions are 32-bit in Thumb2, it makes things easy.
We just need to select the appropriate adjustment.  Do this by moving
the adjustment out of do_undefinstr() into the assembly code, as only
the assembly code knows whether it's dealing with a 32-bit or 16-bit
instruction.

Cc: <stable@vger.kernel.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 11 insertions, 8 deletions

diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
index 2d30c7f6edd3..3a0efaad6090 100644
--- a/arch/arm/vfp/vfphw.S
+++ b/arch/arm/vfp/vfphw.S
@@ -61,13 +61,13 @@
 
 @ VFP hardware support entry point.
 @
-@  r0  = faulted instruction
-@  r2  = faulted PC+4
-@  r9  = successful return
+@  r0  = instruction opcode (32-bit ARM or two 16-bit Thumb)
+@  r2  = PC value to resume execution after successful emulation
+@  r9  = normal "successful" return address
 @  r10 = vfp_state union
 @  r11 = CPU number
-@  lr  = failure return
-
+@  lr  = unrecognised instruction return address
+@  IRQs enabled.
 ENTRY(vfp_support_entry)
         DBGSTR3 "instr %08x pc %08x state %p", r0, r2, r10
 
@@ -161,9 +161,12 @@ vfp_hw_state_valid:
                                         @ exception before retrying branch
                                         @ out before setting an FPEXC that
                                         @ stops us reading stuff
-        VFPFMXR FPEXC, r1               @ restore FPEXC last
-        sub     r2, r2, #4
-        str     r2, [sp, #S_PC]         @ retry the instruction
+        VFPFMXR FPEXC, r1               @ Restore FPEXC last
+        sub     r2, r2, #4              @ Retry current instruction - if Thumb
+        str     r2, [sp, #S_PC]         @ mode it's two 16-bit instructions,
+                                        @ else it's one 32-bit instruction, so
+                                        @ always subtract 4 from the following
+                                        @ instruction address.
 #ifdef CONFIG_PREEMPT
         get_thread_info r10
         ldr     r4, [r10, #TI_PREEMPT]  @ get preempt count