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 S1755019AbbFRKTz (ORCPT <rfc822;w@1wt.eu>);
	Thu, 18 Jun 2015 06:19:55 -0400
Received: from mail-wi0-f176.google.com ([209.85.212.176]:35657 "EHLO
	mail-wi0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753977AbbFRKTr (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Thu, 18 Jun 2015 06:19:47 -0400
Date: Thu, 18 Jun 2015 12:19:43 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker <fweisbec@gmail.com>,
        Rik van Riel <riel@redhat.com>, Oleg Nesterov <oleg@redhat.com>,
        Denys Vlasenko <vda.linux@googlemail.com>,
        Borislav Petkov <bp@alien8.de>, Kees Cook <keescook@chromium.org>,
        Brian Gerst <brgerst@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC/INCOMPLETE 08/13] x86/entry/64: Migrate 64-bit syscalls to
 new exit hooks
Message-ID: <20150618101943.GA6952@gmail.com>
References: <cover.1434485184.git.luto@kernel.org>
 <0ee44a97cf6ffd9a948425b27fa8d48fa271c440.1434485184.git.luto@kernel.org>
 <20150617100009.GA5673@gmail.com>
 <20150617100242.GA6015@gmail.com>
 <CALCETrUzQw=78_3i6=nmLbzw+hW7WtYo2iAaj+7+hH1RD_EaAw@mail.gmail.com>
 <20150618101709.GB6149@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150618101709.GB6149@gmail.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


* Ingo Molnar <mingo@kernel.org> wrote:

> 
> * Andy Lutomirski <luto@amacapital.net> wrote:
> 
> > >> Any reason why irq state tracking cannot be done in C as well, like the 
> > >> rest of the irq state tracking code?
> > >
> > > Never mind, I see you've done exactly that in patch #12.
> > 
> > There are still some TRACE_IRQS_ON, LOCKDEP_SYS_EXIT, and such scattered 
> > throughout the asm.  it's plausible that even more of that could be moved to 
> > C.
> > 
> > We could also benchmark and find out how bad it would be if we just always 
> > filled pt_regs in completely in syscalls.  If the performance hit isn't enough 
> > to matter, then we could potentially move the entire syscall path except 
> > pt_regs setup and sysret/iret into three C functions.
> 
> The thing is, I'd not be against simplifying pt_regs handling even if it slows 
> down things a tiny bit. If anyone wants to reintroduce that complexity we'll see 
> how it looks like in isolation, done cleanly.

... and I suspect the reduction of entry points will allow the compiler to do a 
better job - so some of the overhead might be won back.

So I'd say we try this approach and complicate it back in the future only if the 
numbers warrant it.

Thanks,

	Ingo