[PATCH] powerpc: Fix possible unrecoverable exception caused by '70fe3d9'

[Cyril Bur <cyrilbur@gmail.com>]

Commit 70fe3d9 "powerpc: Restore FPU/VEC/VSX if previously used" introduces a
call to restore_math() late in the syscall return path, after MSR_RI has been
cleared. The MSR_RI flag is used to indicate whether the kernel can take
another exception or not. A cleared MSR_RI flag indicates that the kernel
cannot.

Unfortunately when a machine is under high load an SLB miss can occur in
restore_math() which (with MSR_RI cleared) leads to an unrecoverable exception.

Unrecoverable exception trace:
 powerpc: Restore FPU/VEC/VSX if previously used
  Unrecoverable exception 4100 at c0000000000088d8
  cpu 0x0: Vector: 4100  at [c0000003fa473b20]
      pc: c0000000000088d8: .load_vr_state+0x70/0x110
      lr: c00000000000f710: .restore_math+0x130/0x188
      sp: c0000003fa473da0
     msr: 9000000002003030
    current = 0xc0000007f876f180
    paca    = 0xc00000000fff0000	 softe: 0	 irq_happened: 0x01
      pid   = 1944, comm = K08umountfs
  Linux version 4.5.0-rc3-g22ccd98 (kerkins@alpine1-p1) (gcc version 5.2.1
  20151001 (GCC) ) #1 SMP Tue Mar 15 21:33:26 AEDT 2016
  WARNING: exception is not recoverable, can't continue
  enter ? for help
  [link register   ] c00000000000f710 .restore_math+0x130/0x188
  [c0000003fa473da0] c0000003fa473e30 (unreliable)
  [c0000003fa473e30] c000000000007b6c system_call+0x84/0xfc
  --- Exception: c00 (System Call) at 000000000fe84328
  0:mon>

The clearing of MSR_RI is actually an optimisation to avoid multiple MSR
writes, what must be disabled are interrupts. See comment in entry_64.S:
  /*
   * For performance reasons we clear RI the same time that we
   * clear EE. We only need to clear RI just before we restore r13
   * below, but batching it with EE saves us one expensive mtmsrd call.
   * We have to be careful to restore RI if we branch anywhere from
   * here (eg syscall_exit_work).
   */
At the point of calling restore_math() r13 has not been restored, as such, the
quick fix of turning MSR_RI back on for the call to restore_math() will
eliminate the occurrence of an unrecoverable exception.

We'd like to do a better fix in future.

Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
---
 arch/powerpc/kernel/entry_64.S | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 038e0a1..f3aa4b4 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -218,7 +218,16 @@ system_call:			/* label this so stack traces look sane */
 	bne	3f
 #endif
 2:	addi    r3,r1,STACK_FRAME_OVERHEAD
+#ifdef CONFIG_PPC_BOOK3S
+	mtmsrd	r10,1		/* Restore RI */
+#endif
 	bl	restore_math
+#ifdef CONFIG_PPC_BOOK3S
+	ld	r10,PACAKMSR(r13)
+	li	r9,MSR_RI
+	andc	r11,r10,r9 /* Re-clear RI */
+	mtmsrd	r11,1
+#endif
 	ld	r8,_MSR(r1)
 	ld	r3,RESULT(r1)
 	li	r11,-MAX_ERRNO
-- 
2.7.3