From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="rCFKbF3m" Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3EE811719; Fri, 24 Nov 2023 02:15:47 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=X/ymfIJYGS3rgfsyyKZPvatWK1hMRdU56iYMTo6mr84=; b=rCFKbF3mDACBFhTXzRNdFrFO4O eoXOKmJ9nZiGJbfFMIBv6nwz3CsjBdIoMhWkUoG1xIESyQX9y7utknNb2crx4yYaVl0TokaNQnX59 UQEefO9tha/lsxyx4F65ZPyZ5QCqVo0rlZL/kdkYuCLGIjIenfBE0G4ANfmh4TiCyiHjQw5pCSoOw 9nE6YLG7xrJLF6VvKnJbRf5H0lcpRBYkeFjXyVCwHcJ8GALVBfL/W2eebEDWHFXmQx9wxeHAVLh5J AP7Md+iD5Lbnbp5HhKqJfhCerVvC52VYb0wyb/cpFm3UZZNXbX+76free6tqI6J6N/OKWjkdwAGLS Lh31sbQw==; Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.96 #2 (Red Hat Linux)) id 1r6TDU-00DqKC-0M; Fri, 24 Nov 2023 10:15:20 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 83119300338; Fri, 24 Nov 2023 11:15:19 +0100 (CET) Date: Fri, 24 Nov 2023 11:15:19 +0100 From: Peter Zijlstra To: Christoph Muellner Cc: linux-riscv@lists.infradead.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Palmer Dabbelt , Paul Walmsley , Albert Ou , Andrew Morton , Shuah Khan , Jonathan Corbet , Anup Patel , Philipp Tomsich , Andrew Jones , Guo Ren , Daniel Henrique Barboza , Conor Dooley , =?iso-8859-1?Q?Bj=F6rn_T=F6pel?= , Alan Stern , Andrea Parri , Will Deacon , Daniel Lustig Subject: Re: [RFC PATCH 0/5] RISC-V: Add dynamic TSO support Message-ID: <20231124101519.GP3818@noisy.programming.kicks-ass.net> References: <20231124072142.2786653-1-christoph.muellner@vrull.eu> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20231124072142.2786653-1-christoph.muellner@vrull.eu> On Fri, Nov 24, 2023 at 08:21:37AM +0100, Christoph Muellner wrote: > From: Christoph Müllner > > The upcoming RISC-V Ssdtso specification introduces a bit in the senvcfg > CSR to switch the memory consistency model at run-time from RVWMO to TSO > (and back). The active consistency model can therefore be switched on a > per-hart base and managed by the kernel on a per-process/thread base. You guys, computers are hartless, nobody told ya? > This patch implements basic Ssdtso support and adds a prctl API on top > so that user-space processes can switch to a stronger memory consistency > model (than the kernel was written for) at run-time. > > I am not sure if other architectures support switching the memory > consistency model at run-time, but designing the prctl API in an > arch-independent way allows reusing it in the future. IIRC some Sparc chips could do this, but I don't think anybody ever exposed this to userspace (or used it much). IA64 had planned to do this, except they messed it up and did it the wrong way around (strong first and then relax it later), which lead to the discovery that all existing software broke (d'uh). I think ARM64 approached this problem by adding the load-acquire/store-release instructions and for TSO based code, translate into those (eg. x86 -> arm64 transpilers). IIRC Risc-V actually has such instructions as well, so *why* are you doing this?!?!