From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1753135AbbC0MQz (ORCPT <rfc822;w@1wt.eu>);
	Fri, 27 Mar 2015 08:16:55 -0400
Received: from mail-wg0-f48.google.com ([74.125.82.48]:35175 "EHLO
	mail-wg0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752600AbbC0MQu (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 27 Mar 2015 08:16:50 -0400
Date: Fri, 27 Mar 2015 13:16:45 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Brian Gerst <brgerst@gmail.com>, Andy Lutomirski <luto@amacapital.net>,
        Borislav Petkov <bp@alien8.de>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] x86/asm/entry/64: better check for canonical address
Message-ID: <20150327121645.GC15631@gmail.com>
References: <1427373731-13056-1-git-send-email-dvlasenk@redhat.com>
 <20150327081141.GA9526@gmail.com>
 <551534B1.6090908@redhat.com>
 <20150327111738.GA8749@gmail.com>
 <CAMzpN2hZrL9WLAHrxM4dsa8wkNHWtX6NVGyhRz_MoiED6Q5X8A@mail.gmail.com>
 <20150327113430.GC14778@gmail.com>
 <551549AF.50808@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <551549AF.50808@redhat.com>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org


* Denys Vlasenko <dvlasenk@redhat.com> wrote:

> > Indeed, an IRET ought to be pretty cheap for same-ring interrupt 
> > returns in any case.
> 
> Unfortunately, it is not. Try attached program.
> 
> On this CPU, 1 ns ~= 3 cycles.
> 
> $ ./timing_test64 callret
> 10000 loops in 0.00008s = 7.87 nsec/loop for callret
> 100000 loops in 0.00076s = 7.56 nsec/loop for callret
> 1000000 loops in 0.00548s = 5.48 nsec/loop for callret
> 10000000 loops in 0.02882s = 2.88 nsec/loop for callret
> 100000000 loops in 0.18334s = 1.83 nsec/loop for callret
> 200000000 loops in 0.36051s = 1.80 nsec/loop for callret
> 400000000 loops in 0.71632s = 1.79 nsec/loop for callret
> 
> Near call + near ret = 5 cycles
> 
> $ ./timing_test64 lret
> 10000 loops in 0.00034s = 33.95 nsec/loop for lret
> 100000 loops in 0.00328s = 32.83 nsec/loop for lret
> 1000000 loops in 0.04541s = 45.41 nsec/loop for lret
> 10000000 loops in 0.32130s = 32.13 nsec/loop for lret
> 20000000 loops in 0.64191s = 32.10 nsec/loop for lret
> 
> push my_cs + push next_label + far ret = ~90 cycles
> 
> $ ./timing_test64 iret
> 10000 loops in 0.00344s = 343.90 nsec/loop for iret
> 100000 loops in 0.01890s = 188.97 nsec/loop for iret
> 1000000 loops in 0.08228s = 82.28 nsec/loop for iret
> 10000000 loops in 0.77910s = 77.91 nsec/loop for iret
> 
> This is the "same-ring interrupt return". ~230 cycles!  :(

Ugh, that's really expensive! Why is that so? Same-ring irqs are 
supposedly a lot simpler.

Thanks,

	Ingo