CONFIG_HIGHMEM_START is not available for thedefaultarchitecture x86.
Result is shown for architecture microblaze
default or selected kernelversion does not have config value CONFIG_HIGHMEM_START.
Result is shown for kernelversion 6.2.5

Virtual start address of high memory pool

configname: CONFIG_HIGHMEM_START

Linux Kernel Configuration
└─>Advanced setup
└─>Virtual start address of high memory pool
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
depends
CONFIG_MMU