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SubjectRe: [PATCH 1 / ...] i386 dynamic fixup/self modifying code
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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