Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752919AbbLJFyp (ORCPT ); Thu, 10 Dec 2015 00:54:45 -0500 Received: from mail-ob0-f175.google.com ([209.85.214.175]:35232 "EHLO mail-ob0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751375AbbLJFyo (ORCPT ); Thu, 10 Dec 2015 00:54:44 -0500 MIME-Version: 1.0 In-Reply-To: References: <1449666173-15366-1-git-send-email-brgerst@gmail.com> From: Andy Lutomirski Date: Wed, 9 Dec 2015 21:54:23 -0800 Message-ID: Subject: Re: [PATCH] x86/entry/64: Remove duplicate syscall table for fast path To: Brian Gerst Cc: Andy Lutomirski , "the arch/x86 maintainers" , Linux Kernel Mailing List , Borislav Petkov , =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= , Denys Vlasenko , Linus Torvalds Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1880 Lines: 55 On Wed, Dec 9, 2015 at 9:42 PM, Brian Gerst wrote: > On Wed, Dec 9, 2015 at 6:50 PM, Andy Lutomirski wrote: >> On Wed, Dec 9, 2015 at 1:15 PM, Andy Lutomirski wrote: >>> On Wed, Dec 9, 2015 at 1:08 PM, Brian Gerst wrote: >>>> Simplified version: >>>> ENTRY(stub_ptregs_64) >>>> cmpl $fast_path_return, (%rsp) >>> >>> Does that instruction actually work the way you want it to? (Does it >>> link?) I think you might need to use leaq the way I did in my patch. > > It should have been cmpq. leaq isn't necessary, since immediates are > sign-extended to 64-bit. Right, I always forget that they're sign-extended and not zero-extended. I folded that bit in to my queue. > >>>> jne 1f >>>> SAVE_EXTRA_REGS offset=8 >>>> call *%rax >>>> RESTORE_EXTRA_REGS offset=8 >>>> ret >>>> 1: >>>> jmp *%rax >>>> END(stub_ptregs_64) >>> >>> This'll work, I think, but I still think I prefer keeping as much >>> complexity as possible in the slow path. I could be convinced >>> otherwise, though -- this variant is reasonably clean. >> >> On further reflection, there's at least one functional difference. >> With my variant, modifying pt_regs from sys_foo/ptregs is safe. In >> your variant, it's unsafe unless force_iret() is called. I don't know >> whether we care. > > I can go either way at this point. My main concern was getting rid of > the duplicate table. Agreed. I'll sleep on it, and maybe someone else has some reason to prefer one approach over the other. --Andy -- Andy Lutomirski AMA Capital Management, LLC -- 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/