CONFIG_8xx_COPYBACK is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc
default or selected kernelversion does not have config value CONFIG_8xx_COPYBACK.
Result is shown for kernelversion 6.9.2

Copy-Back Data Cache (else Writethrough)

configname: CONFIG_8xx_COPYBACK

Linux Kernel Configuration
└─>MPC8xx CPM Options
└─>Copy-Back Data Cache (else Writethrough)
In linux kernel since version 2.6.12  
Saying Y here will cause the cache on an MPC8xx processor to be used
in Copy-Back mode. If you say N here, it is used in Writethrough

If in doubt, say Y here.