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Use flags are in the USE variable located at /etc/portage/make.conf.
After changing the USE flags, use this command to recompile all the packages to reflect the new flags:
/usr/src/linux-<version>-gentoomake menuconfig to configure your kernelmake -j$TC && make -j$TC moddules_install && make install (where $TC is the thread count, i.e. the number of cores in your CPU+1)grub-mkconfig -o /boot/grub/grub.cfggenkernel initramfs to update your initramfs