The iasl version present on RHEL-5.4 is very old, and the source isn't even available upstream anymore.
Considering that the only critical use case of iasl is for building the ACPI tables on the KVM package build, and there are existing bugs on the current RHEL-5.4 version, rebasing to a newer upstream version should be safer than keeping the old version on RHEL-5.5.
Some of the bugs on the current iasl version are:
Bug #499540 iasl: "-g" option cannot get ACPI tables
Bug #499552 iasl: Fail to compiling it after disassemble AML file
Bug #500279 iasl: "-a" option is not available
Bug #499772 iasl: Fail to compiling it after disassemble AML file
Bug #518109 Need to update iasl to properly decode DMAR tables
iasl will be updated to version 20090123, from http://www.acpica.org/download/acpica-unix-20090123.tar.gz
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The iasl package was updated to version 20090123, that includes the fix for bug #518109.
An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.