linux-kernel - [PATCH 3.16 078/133] ARC: Re-enable MMU upon Machine Check exception

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Message-ID: <lsq.1511315893.552517455@decadent.org.uk>
Date:   Wed, 22 Nov 2017 01:58:13 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Jose Abreu" <joabreu@...opsys.com>,
        "Vineet Gupta" <vgupta@...opsys.com>,
        "Alexey Brodkin" <abrodkin@...opsys.com>,
        "Jose Abreu" <Jose.Abreu@...opsys.com>
Subject: [PATCH 3.16 078/133] ARC: Re-enable MMU upon Machine Check exception

3.16.51-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jose Abreu <Jose.Abreu@...opsys.com>

commit 1ee55a8f7f6b7ca4c0c59e0b4b4e3584a085c2d3 upstream.

I recently came upon a scenario where I would get a double fault
machine check exception tiriggered by a kernel module.
However the ensuing crash stacktrace (ksym lookup) was not working
correctly.

Turns out that machine check auto-disables MMU while modules are allocated
in kernel vaddr spapce.

This patch re-enables the MMU before start printing the stacktrace
making stacktracing of modules work upon a fatal exception.

Signed-off-by: Jose Abreu <joabreu@...opsys.com>
Reviewed-by: Alexey Brodkin <abrodkin@...opsys.com>
Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
[vgupta: moved code into low level handler to avoid in 2 places]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/arc/kernel/entry.S | 6 ++++++
 arch/arc/mm/tlb.c       | 3 ---
 2 files changed, 6 insertions(+), 3 deletions(-)

--- a/arch/arc/kernel/entry.S
+++ b/arch/arc/kernel/entry.S
@@ -315,6 +315,12 @@ ENTRY(EV_MachineCheck)
 	lr  r0, [efa]
 	mov r1, sp
 
+	; hardware auto-disables MMU, re-enable it to allow kernel vaddr
+	; access for say stack unwinding of modules for crash dumps
+	lr	r3, [ARC_REG_PID]
+	or	r3, r3, MMU_ENABLE
+	sr	r3, [ARC_REG_PID]
+
 	lsr  	r3, r2, 8
 	bmsk 	r3, r3, 7
 	brne    r3, ECR_C_MCHK_DUP_TLB, 1f
--- a/arch/arc/mm/tlb.c
+++ b/arch/arc/mm/tlb.c
@@ -691,9 +691,6 @@ void do_tlb_overlap_fault(unsigned long
 
 	local_irq_save(flags);
 
-	/* re-enable the MMU */
-	write_aux_reg(ARC_REG_PID, MMU_ENABLE | read_aux_reg(ARC_REG_PID));
-
 	/* loop thru all sets of TLB */
 	for (set = 0; set < mmu->sets; set++) {