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x-google-dkim=fail (body has been altered, 2048-bit rsa key) header.d=1e100.net header.i=@1e100.net header.b=ufS4IWNR; x-ptr=pass x-ptr-helo=vger.kernel.org x-ptr-lookup=vger.kernel.org; x-return-mx=pass smtp.domain=vger.kernel.org smtp.result=pass smtp_org.domain=kernel.org smtp_org.result=pass smtp_is_org_domain=no header.domain=sifive.com header.result=pass header_is_org_domain=yes; x-vs=clean score=-100 state=0 X-ME-VSCategory: clean Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751531AbeCTRMJ (ORCPT ); Tue, 20 Mar 2018 13:12:09 -0400 Received: from mail-pf0-f193.google.com ([209.85.192.193]:39891 "EHLO mail-pf0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751502AbeCTRLs (ORCPT ); Tue, 20 Mar 2018 13:11:48 -0400 X-Google-Smtp-Source: AG47ELuMxOFzugm09+HMnP33/rXnHfvMfaq2EX6Yrhp+puW00QaTiWKcDwXN28nuFmRqjJRTfmSNEg== Date: Tue, 20 Mar 2018 10:11:47 -0700 (PDT) X-Google-Original-Date: Tue, 20 Mar 2018 10:01:23 PDT (-0700) Subject: Re: [PATCH 00/16] remove eight obsolete architectures In-Reply-To: CC: hare@suse.de, dhowells@redhat.com, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-block@vger.kernel.org, linux-ide@vger.kernel.org, linux-input@vger.kernel.org, netdev@vger.kernel.org, linux-wireless@vger.kernel.org, linux-pwm@vger.kernel.org, linux-rtc@vger.kernel.org, linux-spi@vger.kernel.org, linux-usb@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org, linux-watchdog@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org From: Palmer Dabbelt To: Arnd Bergmann Message-ID: Mime-Version: 1.0 (MHng) Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-usb-owner@vger.kernel.org X-Mailing-List: linux-usb@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On Thu, 15 Mar 2018 03:42:25 PDT (-0700), Arnd Bergmann wrote: > On Thu, Mar 15, 2018 at 10:59 AM, Hannes Reinecke wrote: >> On 03/15/2018 10:42 AM, David Howells wrote: >>> Do we have anything left that still implements NOMMU? >>> >> RISC-V ? >> (evil grin :-) > > Is anyone producing a chip that includes enough of the Privileged ISA spec > to have things like system calls, but not the MMU parts? > > I thought at least initially the kernel only supports hardware that has a rather > complete feature set. We currently do not have a NOMMU port. As far as I know, everyone who's currently producing RISC-V hardware with enough memory to run Linux has S mode with paging support. The ISA allows for S mode without paging but there's no hardware for that -- if you're going to put a DRAM controller on there then paging seems pretty cheap. You could run a NOMMU port on a system with S-mode and paging, but With all the superpage stuff I don't think you'll get an appreciable performance win for any workload running without an MMU so there's nothing to justify the work (and incompatibility) of a NOMMU port there. While I think you could implement a NOMMU port on a machine with only M and U modes (and therefor no address translation at all), I don't know of any MU-only machines that have enough memory to run Linux (ours have less than 32KiB). A SBI-free Linux would be a prerequisite for this, but there's some interest in that outside of a NOMMU port so it might materialize anyway. Of course, QEMU could probably be tricked into emulating one of these machines with little to no effort :)... That said, I doubt we'll see a NOMMU port materialize without some real hardware as it's a lot of work for a QEMU-only target.