Created attachment 338370 [details] add dolvm to kernel line in /etc/grub.d/10_linux for grub2-mkconfig grub-mkconfig doesn't add "dolvm" to kernel lines when / is on LVM. Attached patch for ebuild that adds this in 10_linux.in (/etc/grub.d/10_linux).
I think that is very specific to the genkernel initramfs. You should probably just add it to GRUB_CMDLINE_LINUX yourself. If you disagree, please submit the patch to grub upstream instead.