How does the kernel know if the CPU is in user mode or kenel mode?

Question

Since the CPU runs in user/kernel mode, I want to know how this is determined by kernel. I mean, if a sys call is invoked, the kernel executes it on behalf of the process, but how does the kernel know that it is executing in kernel mode?

Answer 1

You can tell if you're in user-mode or kernel-mode from the privilege level set in the code-segment register (CS). Every instruction loaded into the CPU from the memory pointed to by the RIP or EIP register (the instruction pointer register depending on if you are x86_64 or x86 respectively) will read from the segment described in the global descriptor table (GDT) by the current code-segment descriptor. The lower two-bits of the code segment descriptor will determine the current privilege level that the code is executing at. When a syscall is made, which is typically done through a software interrupt, the CPU will check the current privilege-level, and if it's in user-mode, will exchange the current code-segment descriptor for a kernel-level one as determined by the syscall's software interrupt gate descriptor, as well as make a stack-switch and save the current flags, the user-level CS value and RIP value on this new kernel-level stack. When the syscall is complete, the user-mode CS value, flags, and instruction pointer (EIP or RIP) value are restored from the kernel-stack, and a stack-switch is made back to the current executing processes' stack.