The hunt for a faster syscall trap
The performance of the syscall trap gets a lot of attention.
I was reminded of a meeting that took place between Intel and
Microsoft over fifteen years ago. (Sadly, I was not myself at this
meeting, so the story is second-hand.)
Since Microsoft is one of
Intel's biggest customers, their representatives often
visit Microsoft to show off what their latest processor can do,
lobby the kernel development team to support a new processor feature,
and solicit feedback on what sort of features would be most useful
to add.
At this meeting, the Intel representatives asked,
"So if you could ask for only one thing to be made faster,
what would it be?"
Without hesitation, one of the lead kernel developers replied,
"Speed up faulting on an invalid instruction."
The Intel half of the room burst out laughing.
"Oh, you Microsoft engineers are so funny!"
And so the meeting ended with a cute little joke.
After returning to their labs, the Intel engineers ran profiles
against the Windows kernel and lo and behold, they discovered
that Windows spent a lot of its time dispatching invalid instruction
exceptions. How absurd! Was the Microsoft engineer not kidding
around after all?
No he wasn't.
It so happens that on the 80386 chip of that era, the fastest
way to get from V86-mode into kernel mode was to execute an invalid
instruction! Consequently, Windows/386 used an invalid instruction
as its syscall trap.
What's the moral of this story? I'm not sure.
Perhaps it's that when you create something,
you may find people using it in ways you had never considered.
