A simple Linux kernel module which, when loaded, allows ring 3 (userspace) programs to use the RDPMC assembler instruction by flipping the PMC flag in the CR4 register.

This module only makes sense on an x86 machine.

Install Linux kernel headers and run make.

The build currently assumes a Debian Linux system. You may have to tweak some paths in the Makefile for other distributions.

First run the test_rdpmc program and check that the program crashes, thus confirming that the userspace cannot invoke RDPMC.

Insert the module with sudo insmod user_rdpmc.ko. The dmesg buffer should then display a message similar to:

[ 5016.743382] Enabling RDPMC from ring 3 for 4 CPUs

Now if you re-run the test program, it should not crash. Userspace can now invoke RDPMC.

If you remove the user_rdpmc module, userspace is then denied use of RDPMC.