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 S1755290AbbCaQno (ORCPT <rfc822;w@1wt.eu>);
	Tue, 31 Mar 2015 12:43:44 -0400
Received: from mail-wg0-f42.google.com ([74.125.82.42]:34587 "EHLO
	mail-wg0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753192AbbCaQnn (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Tue, 31 Mar 2015 12:43:43 -0400
Date: Tue, 31 Mar 2015 18:43:37 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>, 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: <20150331164337.GA8462@gmail.com>
References: <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>
 <20150327121645.GC15631@gmail.com>
 <55154DB3.9000008@redhat.com>
 <20150328091106.GA5361@gmail.com>
 <CAK1hOcOOQAq1sWdnK+nTZBightFaJWiTeQCmCPzhG0-y-UYvOw@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAK1hOcOOQAq1sWdnK+nTZBightFaJWiTeQCmCPzhG0-y-UYvOw@mail.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


* Denys Vlasenko <vda.linux@googlemail.com> wrote:

> > I guess they could optimize it by adding a single "I am a modern 
> > OS executing regular userspace" flag to the descriptor [or 
> > expressing the same as a separate instruction], to avoid all that 
> > legacy crap that won't trigger on like 99.999999% of systems ...
> 
> Yes, that would be a useful addition. Interrupt servicing on x86 
> takes a non-negligible hit because of IRET slowness.

But ... to react to your other patch: detecting the common easy case 
and doing a POPF+RET ourselves ought to be pretty good as well?

But only if ptregs->rip != the magic RET itself, to avoid recursion.

Even with all those extra checks it should still be much faster.

Thanks,

	Ingo