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Re: [PATCH] Avoid potential hazard on Context register

To: Chris Dearman <>
Subject: Re: [PATCH] Avoid potential hazard on Context register
From: Ralf Baechle <>
Date: Sun, 11 Oct 2009 16:53:30 +0200
Cc: linux-mips <>
In-reply-to: <>
Original-recipient: rfc822;
References: <> <>
User-agent: Mutt/1.5.19 (2009-01-05)
There is no hazard barrier between writes to c0_context and subsequent
read accesses.  This is a fairly theoretical hole as c0_context is only
written on CPU bootup and other, unrelated code will almost certainly
execute a hazard barrier somewhen between the write and read access.
Even if not, the window is probably in the thousands of cycles so likely
too large to actually consistute a pipeline hazard.

Reported and initial patch by Chris Dearman <>.

Signed-off-by: Ralf Baechle <>

 arch/mips/include/asm/mmu_context.h |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/arch/mips/include/asm/mmu_context.h 
index ed331c2..6083db5 100644
--- a/arch/mips/include/asm/mmu_context.h
+++ b/arch/mips/include/asm/mmu_context.h
@@ -16,6 +16,7 @@
 #include <linux/smp.h>
 #include <linux/slab.h>
 #include <asm/cacheflush.h>
+#include <asm/hazards.h>
 #include <asm/tlbflush.h>
 #include <asm/mipsmtregs.h>
@@ -36,11 +37,13 @@ extern unsigned long pgd_current[];
 #ifdef CONFIG_32BIT
 #define TLBMISS_HANDLER_SETUP()                                                
        write_c0_context((unsigned long) smp_processor_id() << 25);     \
+       back_to_back_c0_hazard();                                       \
 #ifdef CONFIG_64BIT
 #define TLBMISS_HANDLER_SETUP()                                                
        write_c0_context((unsigned long) smp_processor_id() << 26);     \
+       back_to_back_c0_hazard();                                       \

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