linux-kernel - For your amusement: slightly faster syscalls

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Message-ID: <CALCETrXUiB9fCRERnKOpcsxmsm44qWw7vnk42j38B4+bJm0=RA@mail.gmail.com>
Date:	Fri, 12 Jun 2015 17:09:07 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Borislav Petkov <bp@...en8.de>
Subject: For your amusement: slightly faster syscalls

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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