Take it you have done the following tweaks allready.

GRUB_CMDLINE_LINUX="msr.allow_writes=on" in /etc/default/grub and update-grub.
As normally msr writes are not allowed. Also need to have msr-tools installed as well.

To boost xmrig get the following script from the xmrig/scripts directory on github:
randomx_boost.sh and and maybe enable_1gb_pages.sh run them as root or w/sudo.
In the config.json file change "rdmsr": true, and "wrmsr": true, both to false

and then you can run xmrig as user otherwise you won't benefit as most tweaks you need to run xmrig under root w/sudo.

Same actually counts for SRBMiner-MULTI and not only those miners but practically all miners.