For your amusement: slightly faster syscalls

From	Andy Lutomirski <luto@amacapital.net>
Newsgroups	linux.kernel
Subject	For your amusement: slightly faster syscalls
Date	2015-06-13 02:10 +0200
Message-ID	<pAHbj-4H5-7@gated-at.bofh.it> (permalink)
Organization	linux.* mail to news gateway

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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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For your amusement: slightly faster syscalls Andy Lutomirski <luto@amacapital.net> - 2015-06-13 02:10 +0200
  Re: For your amusement: slightly faster syscalls Ingo Molnar <mingo@kernel.org> - 2015-06-18 10:10 +0200

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