The registers that are saved/restored in the trap / exception frame need to be consistent between all entry/exit points as well as the functions that convert between trap/exception frame and context. The trap frame contains only the non-volatile registers and rbp, the rest is saved in the exception frame.
The previous code didn't save rbp in the syscall handler, which led to it being clobbered when exiting though KiServiceExit2 rather than returning back to the syscall exit path. Also KeContextToTrapFrame would use rbx, rsi and rdi from the trap frame, which wouldn't be saved there by the syscall handler.
KiSetTrapContext is an asm wrapper around RtlSetUnwindContext, which first stores an exception frame to assure that all non-volatile registers were put on the stack, then calls RtlSetUnwindContext to update their first saving positions on the stack and finally restore the exception frame to potentially load any updated registers, that haven't been saved elsewhere on the stack.
[REACTOS] Misc 64 bit fixes
* [NTOS:MM] Allow MEM_DOS_LIM in NtMapViewOfSection on x64 as well
* [NTOS:MM] Implement x64 version of MmIsDisabledPage
* [HAL] Remove obsolete code
* [NTOS:KE] Fix amd64 version of KeContextToTrapFrame and KeTrapFrameToContext
* [XDK] Fix CONTEXT_XSTATE definition
* [PCNET] Convert physical address types from pointers to PHYSICAL_ADDRESS
