I was just reading What is linux-gate.so.1?, and it says this:
Intel recognized this problem early on and introduced a more efficient system call interface in the form of
sysexitinstructions. This fast system call feature first appeared in the Pentium Pro processor, but due to hardware bugs it's actually broken in most of the early CPUs
I did some research and it seems as if
SYSENTER was not documented until the Pentium II, and further, this bug that stopped its adoption until much later is hinted at again:
It seems likely that Intel discovered some problem with the
SYSENTERinstruction very shortly before the chip was released. And instead of documenting the problems and fixing them, Intel decided not to document the instruction at all.
The comments on the latter article suggest that the issue might lie in
SYSEXIT rather than
SYSENTER. What specific defects in the Pentium Pro and early Pentium II make it impossible to use
SYSENTER as intended?