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

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Date:	Sun, 1 Apr 2012 08:50:19 +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

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 ?

Thanks,
Willy

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