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[patch] linux: A critical DECstation interrupt handler fix

To: Ralf Baechle <>
Subject: [patch] linux: A critical DECstation interrupt handler fix
From: "Maciej W. Rozycki" <>
Date: Mon, 26 Nov 2001 15:35:17 +0100 (MET)
Organization: Technical University of Gdansk

 The DECstation's interrupt handler can crash under certain circumstances. 
Due to a missing masking of the CP0 Cause register, if a spurious
interrupt is delivered (its deasserted before reading the register), the
handler may jump to an arbitrary memory location as determined by data
fetched from an incorrect location.  Due to this problem my new /260
system used to crash frequently, because Cause.CE is often set to 3 (CE is
unspecified for all but coprocessor unusable exceptions). 

 The following patch masks Cause appropriately.  A small reorganization of
code was also possible due to changes in the scheduling of delay slots. 


+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+        e-mail:, PGP key available        +

diff -up --recursive --new-file 
--- linux-mips-2.4.14-20011123.macro/arch/mips/dec/int-handler.S        Tue Jul 
 3 04:27:16 2001
+++ linux-mips-2.4.14-20011123/arch/mips/dec/int-handler.S      Sun Nov 25 
00:40:11 2001
@@ -140,7 +140,7 @@
                mfc0    t0,CP0_CAUSE            # get pending interrupts
                mfc0    t2,CP0_STATUS
-               la      t1,cpu_mask_tbl
+               andi    t0,ST0_IM               # CAUSE.CE may be non-zero!
                and     t0,t2                   # isolate allowed ones
                beqz    t0,spurious
@@ -148,7 +148,8 @@
                 * Find irq with highest priority
-1:              lw     t2,(t1)
+                la     t1,cpu_mask_tbl
+1:             lw      t2,(t1)
                move    t3,t0
                and     t3,t2
                beq     t3,zero,1b

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