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From:   Ravi Bangoria <ravi.bangoria@linux.ibm.com>
To:     mpe@ellerman.id.au
Cc:     mikey@neuling.org, benh@kernel.crashing.org, paulus@samba.org,
        npiggin@gmail.com, christophe.leroy@c-s.fr,
        mahesh@linux.vnet.ibm.com, linuxppc-dev@lists.ozlabs.org,
        linux-kernel@vger.kernel.org,
        Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Subject: [PATCH] Powerpc/Watchpoint: Restore nvgprs while returning from exception
Date:   Thu,  6 Jun 2019 12:59:51 +0530
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Message-Id: <20190606072951.32116-1-ravi.bangoria@linux.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Powerpc hw triggers watchpoint before executing the instruction.
To make trigger-after-execute behavior, kernel emulates the
instruction. If the instruction is 'load something into non-
volatile register', exception handler should restore emulated
register state while returning back, otherwise there will be
register state corruption. Ex, Adding a watchpoint on a list
can corrput the list:

  # cat /proc/kallsyms | grep kthread_create_list
  c00000000121c8b8 d kthread_create_list

Add watchpoint on kthread_create_list->next:

  # perf record -e mem:0xc00000000121c8c0

Run some workload such that new kthread gets invoked. Ex, I
just logged out from console:

  list_add corruption. next->prev should be prev (c000000001214e00), \
	but was c00000000121c8b8. (next=c00000000121c8b8).
  WARNING: CPU: 59 PID: 309 at lib/list_debug.c:25 __list_add_valid+0xb4/0xc0
  CPU: 59 PID: 309 Comm: kworker/59:0 Kdump: loaded Not tainted 5.1.0-rc7+ #69
  ...
  NIP __list_add_valid+0xb4/0xc0
  LR __list_add_valid+0xb0/0xc0
  Call Trace:
  __list_add_valid+0xb0/0xc0 (unreliable)
  __kthread_create_on_node+0xe0/0x260
  kthread_create_on_node+0x34/0x50
  create_worker+0xe8/0x260
  worker_thread+0x444/0x560
  kthread+0x160/0x1a0
  ret_from_kernel_thread+0x5c/0x70

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
---
 arch/powerpc/kernel/exceptions-64s.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index 9481a11..96de0d1 100644
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S
@@ -1753,7 +1753,7 @@ handle_dabr_fault:
 	ld      r5,_DSISR(r1)
 	addi    r3,r1,STACK_FRAME_OVERHEAD
 	bl      do_break
-12:	b       ret_from_except_lite
+12:	b       ret_from_except
 
 
 #ifdef CONFIG_PPC_BOOK3S_64
-- 
1.8.3.1