You cannot enter ring 0 with any particular instruction under any modern operating system because that is considered security risk and the kernel does not allow your process to execute such instructions. However, you can write a device driver or use already existing driver from within your application to perform privileged operations such as access to MSR registers.
In order to access unique MSR registers your application needs to change its process/thread affinity for each CPU whose MSR you want to read/write.
Under Windows that is accomplished by calling SetThreadAffinityMask() API but for QNX you will have to consult proper OS API documentation or ask someone else.
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