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Some updates to SCR can cause a problem which manifests as an undefined opcode exception. This may be when a speculative secure instruction fetch happens after the NS bit is set. An isb is required to make the register change take effect fully. Contributed-under: Tianocore Contribution Agreement 1.0 Signed-off-by: Evan Lloyd <Evan.Lloyd@arm.com> Reviewed-by: Sami Mujawar <Sami.Mujawar@arm.com> Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>