Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753074AbbDFSHq (ORCPT ); Mon, 6 Apr 2015 14:07:46 -0400 Received: from ppsw-33.csi.cam.ac.uk ([131.111.8.133]:53732 "EHLO ppsw-33.csi.cam.ac.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751292AbbDFSHo (ORCPT ); Mon, 6 Apr 2015 14:07:44 -0400 X-Greylist: delayed 1388 seconds by postgrey-1.27 at vger.kernel.org; Mon, 06 Apr 2015 14:07:43 EDT X-Cam-AntiVirus: no malware found X-Cam-ScannerInfo: http://www.cam.ac.uk/cs/email/scanner/ Message-ID: <5522C5EA.8060803@citrix.com> Date: Mon, 06 Apr 2015 18:44:10 +0100 From: Andrew Cooper User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0 MIME-Version: 1.0 To: Andy Lutomirski , Konrad Rzeszutek Wilk CC: Denys Vlasenko , Denys Vlasenko , X86 ML , Linux Kernel Mailing List , Borislav Petkov , Andy Lutomirski , "H. Peter Anvin" , "xen-devel@lists.xenproject.org" , Boris Ostrovsky , Linus Torvalds , Ingo Molnar Subject: Re: [Xen-devel] [PATCH] x86, paravirt, xen: Remove the 64-bit irq_enable_sysexit pvop References: <8a03355698fe5b94194e9e7360f19f91c1b2cf1f.1428100853.git.luto@kernel.org> <20150406141051.GF12596@l.oracle.com> In-Reply-To: Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1577 Lines: 35 On 06/04/2015 16:29, Andy Lutomirski wrote: > On Mon, Apr 6, 2015 at 7:10 AM, Konrad Rzeszutek Wilk > wrote: >> On Fri, Apr 03, 2015 at 03:52:30PM -0700, Andy Lutomirski wrote: >>> [cc: Boris and Konrad. Whoops] >>> >>> On Fri, Apr 3, 2015 at 3:51 PM, Andy Lutomirski wrote: >>>> We don't use irq_enable_sysexit on 64-bit kernels any more. Remove >> Is there an commit (or name of patch) that explains why 32-bit-user-space-on-64-bit >> kernels is unsavory? > sysexit never tasted very good :-p > > We're (hopefully) not breaking 32-bit-user-space-on-64-bit, but we're > trying an unconventional approach to making the code faster and less > scary. As a result, 64-bit kernels won't use sysexit any more. > Hopefully Xen is okay with the slightly sneaky thing we're doing. > AFAICT Xen thinks of sysretl and sysexit as slightly funny irets, so I > don't expect there to be any problem. 64bit PV kernels must bounce through Xen to switch from the kernel to the user pagetables (since both kernel and userspace are both actually running in ring3 with user pages). As a result, exit to userspace ends up as a hypercall into Xen which has an effect very similar to an `iret`, but with some extra fixup in the background. I can't forsee any Xen issues as a result of this patch. ~Andrew -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/