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From: Mikael Pettersson <mikpe@csd.uu.se>
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Date: Wed, 28 Aug 2002 17:53:11 +0200
To: Luca Barbieri <ldb@ldb.ods.org>
Cc: Linux-Kernel ML <linux-kernel@vger.kernel.org>,
       Linus Torvalds <torvalds@transmeta.com>
Subject: Re: [PATCH 1 / ...] i386 dynamic fixup/self modifying code
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Luca Barbieri writes:
 > This patch implements a system that modifies the kernel code at runtime
 > depending on CPU features and SMPness.
 >...
 > /* If we are running on SMP any other processor might be executing the
 >    code that we are modifying. We must make sure that the other
 >    processor will either fault or will execute the complete
 >    replacement instruction.
 > 
 >    This is accomplished by using instructions that fault only
 >    depending on up to 4 bytes. When fixing up something we first write
 >    bytes after the first 4 and we then use a locked write to set the
 >    first 4.
 > 
 >    We depend on the processor execute unit to never see our locked
 >    write before it sees the other modifications.
 >    
 >    According to page 7-5 of the Intel Pentium 4 System Programming
 >    Manual, this is safe on 486 and Pentium <= 4.

I've tried this sort of thing before (unsynchronised cross-modifying code),
but I had to abandon it due to Pentium III Erratum E49 and similar errata
for all Intel P6 CPUs. Have you verified that you're not hitting this erratum?

/Mikael
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