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

[Mikael Pettersson]

Luca Barbieri writes:
 > > 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?
 > It is indeed completely hitting it.
 > However, we can work around this by simply stopping all other CPUs in
 > interrupt context with an IPI (while this may sound horrible, it
 > shouldn't significantly impact performance unless the response time is
 > excessively long).

That was my thought too. IPI to bring the others to a barrier, do the
modification, release the barrier.

In my case (patching CALL instructions to call the correct targets
after HW detection) I was fortunately able to fix up the code before
it was seen by other CPUs, but this relied on the fact that I knew
the locations of all CALL sites needing fix up.

 > I'll write some code to this. However I don't have the hardware to test
 > it, so it might require multiple iterations to get it right.
 > 
 > As for the "all Intel P6 CPUs" are really _all_ Intel P6 CPU broken? 

Yes, last time I checked the erratum existed for all members of
Intel's P6 family.

 > Do you know of any other CPU that would need the workaround?

No. The P5 is ok, and I believe the P4 is also. The K7s didn't have
this listed as an erratum last time I checked.

/Mikael