We propose a method for dynamic security domain scaling on SMPs that offers both highly scalable performance and high security for future high-end embedded systems. Its most important feature is its highly efficient use of processor resources, accomplished by dynamically changing the number of processors within a security domain in response to application load requirements. Two new technologies make this scaling possible without any virtualization software: 1) self-transition management and 2) unified virtual address mapping. Evaluations show that this domain control provides highly scalable performance and incurs almost no performance overhead in security domains. The increase in binary code size is less than 40KB, and the time required for individual state transitions is of a single-millisecond order. This scaling is the first in the world to make possible dynamic changing of the number of processors within a security domain on an ARM SMP.