Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756163AbbLHNHz (ORCPT ); Tue, 8 Dec 2015 08:07:55 -0500 Received: from mail-vk0-f45.google.com ([209.85.213.45]:33875 "EHLO mail-vk0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750901AbbLHNHx (ORCPT ); Tue, 8 Dec 2015 08:07:53 -0500 MIME-Version: 1.0 In-Reply-To: References: <2ff015fa6989c6a8907c73636f5f5cb99402f6c3.1449522077.git.luto@kernel.org>

Date: Tue, 8 Dec 2015 08:07:52 -0500 Message-ID: Subject: Re: [PATCH 07/12] x86/entry/64: Always run ptregs-using syscalls on the slow path From: Brian Gerst To: Andy Lutomirski 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: 2879 Lines: 71 On Mon, Dec 7, 2015 at 8:12 PM, Andy Lutomirski wrote: > 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. I'm working on a full patch. It will set the flag (renamed TS_SLOWPATH) in do_syscall_64(), which is the only place these functions can get called from C code. Your changes already have it set up so that the slow path saved these registers before calling any C code. Where else do you expect them to be called from? > 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. The syscall table is huge. 545*8 bytes, over a full page. Duplicating it for just a few different entries is wasteful. -- Brian Gerst -- 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/