For your amusement: slightly faster syscalls

[Andy Lutomirski]

The SYSCALL prologue starts with SWAPGS immediately followed by a
gs-prefixed instruction.  I think this causes a pipeline stall.

If we instead do:

mov %rsp, rsp_scratch(%rip)
mov sp0(%rip), %rsp)
swapgs
...
pushq rsp_scratch(%rip)

then we avoid the stall and save about three cycles.

Horrible horrible code to do this lives here:

https://git.kernel.org/cgit/linux/kernel/git/luto/devel.git/log/?h=x86/faster_syscalls

Caveat emptor: it also disables SMP.

For three cycles, I don't think this is worth trying to clean up.

--Andy
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