Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id EF0BEC61DA4 for ; Thu, 16 Feb 2023 07:56:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229704AbjBPH41 (ORCPT ); Thu, 16 Feb 2023 02:56:27 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57348 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229534AbjBPH4Y (ORCPT ); Thu, 16 Feb 2023 02:56:24 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C991242BFA for ; Wed, 15 Feb 2023 23:55:32 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 2AFEBB825D5 for ; Thu, 16 Feb 2023 07:54:45 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 63B89C433EF; Thu, 16 Feb 2023 07:54:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1676534083; bh=KA/5JF5HlerkurForNO9JKH5IFf6vgsN6Wv08r1dIwM=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=l/WJ5eYYPOVw4F5vwb0QTZCVSVofcMD8FP6A6jZfcJsTs2qNkbEuZntflodp7jdjM U395s6xJr3Ll2QU3lJST1ALbt4JjCwjremXGiRU5dIC5tj5OdJJfnKsDviK988ESrg ubh/YJjD7lWxQj2S7uVO+AvKs+5wnP6RqHzSuzYIbtwrklFdnJwaooLHvzXNchiCJV oPe8Uf9t6Lpb5xP8xV1MpO0NN298ErBhPmfmz652cxGdDFBHhAKM+N1HDdJ2Vnb4w+ GVi7KzDV0DsuEtITSI1/EX3SBp/6eBaCipwNuoVWfckGLCHbaANlCeHa4zKCVfl9Nj lFEOtMN0c9oCQ== From: =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= To: Guo Ren , Andrea Parri Cc: "liaochang (A)" , palmer@dabbelt.com, paul.walmsley@sifive.com, mhiramat@kernel.org, conor.dooley@microchip.com, penberg@kernel.org, mark.rutland@arm.com, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, Guo Ren , Changbin Du Subject: Re: [PATCH] riscv: kprobe: Optimize kprobe with accurate atomicity In-Reply-To: References: <20230126161559.1467374-1-guoren@kernel.org> <0abbbdd4-6b85-9659-03ee-97c56a5b77c1@huawei.com> <87r0vc9h4g.fsf@all.your.base.are.belong.to.us> <87ilgntdef.fsf@all.your.base.are.belong.to.us> Date: Thu, 16 Feb 2023 08:54:41 +0100 Message-ID: <878rgy3v0e.fsf@all.your.base.are.belong.to.us> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Guo Ren writes: > On Tue, Jan 31, 2023 at 6:57 PM Andrea Parri wro= te: >> >> > > It's the concurrent modification that I was referring to (removing >> > > stop_machine()). You're saying "it'll always work", I'm saying "I'm = not >> > > so sure". :-) E.g., writing c.ebreak on an 32b insn. Can you say that >> > Software must ensure write c.ebreak on the head of an 32b insn. >> > >> > That means IFU only see: >> > - c.ebreak + broken/illegal insn. >> > or >> > - origin insn >> > >> > Even in the worst case, such as IFU fetches instructions one by one: >> > If the IFU gets the origin insn, it will skip the broken/illegal insn. >> > If the IFU gets the c.ebreak + broken/illegal insn, then an ebreak >> > exception is raised. >> > >> > Because c.ebreak would raise an exception, I don't see any problem. >> >> That's the problem, this discussion is: >> >> Reviewer: "I'm not sure, that's not written in our spec" >> Submitter: "I said it, it's called -accurate atomicity-" > I really don't see any hardware that could break the atomicity of this > c.ebreak scenario: > - c.ebreak on the head of 32b insn > - ebreak on an aligned 32b insn > > If IFU fetches with cacheline, all is atomicity. > If IFU fetches with 16bit one by one, the first c.ebreak would raise > an exception and skip the next broke/illegal instruction. > Even if IFU fetches without any sequence, the IDU must decode one by > one, right? The first half c.ebreak would protect and prevent the next > broke/illegal instruction. Speculative execution on broke/illegal > instruction won't cause any exceptions. > > It's a common issue, not a specific ISA issue. > 32b instruction A -> 16b ebreak + 16b broken/illegal -> 32b > instruction A. It's safe to transform. Waking up this thread again, now that Changbin has showed some interest from another thread [1]. Guo, we can't really add your patches, and claim that they're generic, "works on all" RISC-V systems. While it might work for your I/D coherent system, that does not imply that it'll work on all platforms. RISC-V allows for implementations that are I/D incoherent, and here your IFU-implementations arguments do not hold. I'd really recommend to readup on [2]. Now how could we move on with your patches? Get it in a spec, or fold the patches in as a Kconfig.socs-thing for the platforms where this is OK. What are you thoughts on the latter? Bj=C3=B6rn [1] https://lore.kernel.org/linux-riscv/20230215034532.xs726l7mp6xlnkdf@M91= 0t/ [2] https://github.com/riscv/riscv-j-extension/blob/master/id-consistency-p= roposal.pdf