From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_2 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id CA275C433DF for ; Tue, 21 Jul 2020 23:16:06 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 7EF5520709 for ; Tue, 21 Jul 2020 23:16:06 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7EF5520709 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from bilbo.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 4BBDx040L1zDqjg for ; Wed, 22 Jul 2020 09:16:04 +1000 (AEST) Authentication-Results: lists.ozlabs.org; spf=permerror (SPF Permanent Error: Unknown mechanism found: ip:192.40.192.88/32) smtp.mailfrom=kernel.crashing.org (client-ip=76.164.61.194; helo=kernel.crashing.org; envelope-from=benh@kernel.crashing.org; receiver=) Authentication-Results: lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Received: from kernel.crashing.org (kernel.crashing.org [76.164.61.194]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4BBDt72DwnzDqkR for ; Wed, 22 Jul 2020 09:13:35 +1000 (AEST) Received: from localhost (gate.crashing.org [63.228.1.57]) (authenticated bits=0) by kernel.crashing.org (8.14.7/8.14.7) with ESMTP id 06LNCxrb017185 (version=TLSv1/SSLv3 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NO); Tue, 21 Jul 2020 18:13:05 -0500 Message-ID: <6fbea8347bdb8434d91cf3ec2b95b134bd66cfe3.camel@kernel.crashing.org> Subject: Re: [PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone From: Benjamin Herrenschmidt To: Palmer Dabbelt , alex@ghiti.fr Date: Wed, 22 Jul 2020 09:12:58 +1000 In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.28.5-0ubuntu0.18.04.2 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: aou@eecs.berkeley.edu, linux-mm@kvack.org, Anup Patel , linux-kernel@vger.kernel.org, Atish Patra , paulus@samba.org, zong.li@sifive.com, Paul Walmsley , linux-riscv@lists.infradead.org, linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Tue, 2020-07-21 at 12:05 -0700, Palmer Dabbelt wrote: > > * We waste vmalloc space on 32-bit systems, where there isn't a lot of it. > * On 64-bit systems the VA space around the kernel is precious because it's the > only place we can place text (modules, BPF, whatever). Why ? Branch distance limits ? You can't use trampolines ? > If we start putting > the kernel in the vmalloc space then we either have to pre-allocate a bunch > of space around it (essentially making it a fixed mapping anyway) or it > becomes likely that we won't be able to find space for modules as they're > loaded into running systems. I dislike the kernel being in the vmalloc space (see my other email) but I don't understand the specific issue with modules. > * Relying on a relocatable kernel for sv48 support introduces a fairly large > performance hit. Out of curiosity why would relocatable kernels introduce a significant hit ? Where about do you see the overhead coming from ? > Roughly, my proposal would be to: > > * Leave the 32-bit memory map alone. On 32-bit systems we can load modules > anywhere and we only have one VA width, so we're not really solving any > problems with these changes. > * Staticly allocate a 2GiB portion of the VA space for all our text, as its own > region. We'd link/relocate the kernel here instead of around PAGE_OFFSET, > which would decouple the kernel from the physical memory layout of the system. > This would have the side effect of sorting out a bunch of bootloader headaches > that we currently have. > * Sort out how to maintain a linear map as the canonical hole moves around > between the VA widths without adding a bunch of overhead to the virt2phys and > friends. This is probably going to be the trickiest part, but I think if we > just change the page table code to essentially lie about VAs when an sv39 > system runs an sv48+sv39 kernel we could make it work -- there'd be some > logical complexity involved, but it would remain fast. > > This doesn't solve the problem of virtually relocatable kernels, but it does > let us decouple that from the sv48 stuff. It also lets us stop relying on a > fixed physical address the kernel is loaded into, which is another thing I > don't like. > > I know this may be a more complicated approach, but there aren't any sv48 > systems around right now so I just don't see the rush to support them, > particularly when there's a cost to what already exists (for those who haven't > been watching, so far all the sv48 patch sets have imposed a significant > performance penalty on all systems).