Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932380AbbLHBMb (ORCPT ); Mon, 7 Dec 2015 20:12:31 -0500 Received: from mail-ob0-f177.google.com ([209.85.214.177]:33454 "EHLO mail-ob0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932114AbbLHBM3 (ORCPT ); Mon, 7 Dec 2015 20:12:29 -0500 MIME-Version: 1.0 In-Reply-To: References: <2ff015fa6989c6a8907c73636f5f5cb99402f6c3.1449522077.git.luto@kernel.org> From: Andy Lutomirski Date: Mon, 7 Dec 2015 17:12:09 -0800 Message-ID: Subject: Re: [PATCH 07/12] x86/entry/64: Always run ptregs-using syscalls on the slow 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: 2291 Lines: 60 On Mon, Dec 7, 2015 at 4:54 PM, Brian Gerst wrote: > On Mon, Dec 7, 2015 at 7:50 PM, Brian Gerst wrote: >> On Mon, Dec 7, 2015 at 4:51 PM, Andy Lutomirski wrote: >>> 64-bit syscalls currently have an optimization in which they are >>> called with partial pt_regs. A small handful require full pt_regs. >>> >>> In the 32-bit and compat cases, I cleaned this up by forcing full >>> pt_regs for all syscalls. The performance hit doesn't really matter. >>> >>> I want to clean up the 64-bit case as well, but I don't want to hurt >>> fast path performance. To do that, I want to force the syscalls >>> that use pt_regs onto the slow path. This will enable us to make >>> slow path syscalls be real ABI-compliant C functions. >>> >>> Use the new syscall entry qualification machinery for this. >>> stub_clone is now stub_clone/ptregs. >>> >>> The next patch will eliminate the stubs, and we'll just have >>> sys_clone/ptregs. > > [Resend after gmail web interface fail] > > I've got an idea on how to do this without the duplicate syscall table. > > ptregs_foo: > leaq sys_foo(%rip), %rax > jmp stub_ptregs_64 > > stub_ptregs_64: > testl $TS_EXTRAREGS, ti_status> > jnz 1f > SAVE_EXTRA_REGS > call *%rax > RESTORE_EXTRA_REGS > ret > 1: > call *%rax > ret > > This makes sure that the extra regs don't get saved a second time if > coming in from the slow path, but preserves the fast path if not > tracing. I think there's value in having the entries in the table be genuine C ABI-compliant function pointers. In your example, it only barely works -- you can call them from C only if you have TS_EXTRAREGS set appropriately -- -otherwise you crash and burn. That will break the rest of the series. We could adjust it a bit and check whether we're in C land (by checking rsp for ts) and jump into the slow path if we aren't, but I'm not sure this is a huge win. It does save some rodata space by avoiding duplicating the table. --Andy -- 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/