linux-kernel - Re: [PATCH] x86-32: A better system call mechanism

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Message-ID: <20120401065235.GB27590@1wt.eu>
Date:	Sun, 1 Apr 2012 08:52:35 +0200
From:	Willy Tarreau <w@....eu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH] x86-32: A better system call mechanism

On Sun, Apr 01, 2012 at 08:50:19AM +0200, Willy Tarreau wrote:
> Hi Peter,
> 
> On Sun, Apr 01, 2012 at 12:00:00AM +0000, H. Peter Anvin wrote:
> > On x86-32, we currently use int $0x80 as the primary system call
> > mechanism.  Although there are some recent variants available on
> > certain hardware (sysenter, syscall) via the vdso, the primary system
> > call vector is still way up the interrupt vector table, which is
> > inefficient.
> > 
> > This patch adds a very small amount of code which permits the very
> > first vector to be used for system call.  That vector is #DE, divide
> > error, generally known as division by zero.
> 
> Looks like a clever trick, but beyond the beauty, what does it really
> save ? Code size is the same as aam 0 / div edx are both 2-byte long,
> just like int 0x80. Is the call less expensive ? And if so, how does
> it compare to vdso ?

Hmmm I think I just found the response in the Date header, time to get
some coffee :-)

Willy

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