Fun fact, not only can you use iret to return to user-mode from kernel-mode and kernel-mode from kernel-mode (as footnote 5 mentions), you can also use iret to return from user-mode to user-mode. It is a handy way to restore RIP, RSP, and RFLAGS at the same time. .NET CoreCLR uses it during exception handling on Unix:

https://github.com/dotnet/runtime/blob/4db72366e8e49c30d7aa6...