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 842CBC433DF for ; Wed, 22 Jul 2020 02:22:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6765F20787 for ; Wed, 22 Jul 2020 02:22:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731754AbgGVCWP (ORCPT ); Tue, 21 Jul 2020 22:22:15 -0400 Received: from kernel.crashing.org ([76.164.61.194]:41780 "EHLO kernel.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731595AbgGVCWP (ORCPT ); Tue, 21 Jul 2020 22:22:15 -0400 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 06M2LIGX018560 (version=TLSv1/SSLv3 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NO); Tue, 21 Jul 2020 21:21:25 -0500 Message-ID: Subject: Re: [PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone From: Benjamin Herrenschmidt To: Palmer Dabbelt Cc: alex@ghiti.fr, mpe@ellerman.id.au, paulus@samba.org, Paul Walmsley , aou@eecs.berkeley.edu, Anup Patel , Atish Patra , zong.li@sifive.com, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-mm@kvack.org Date: Wed, 22 Jul 2020 12:21:16 +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 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2020-07-21 at 16:48 -0700, Palmer Dabbelt wrote: > > Why ? Branch distance limits ? You can't use trampolines ? > > Nothing fundamental, it's just that we don't have a large code model in the C > compiler. As a result all the global symbols are resolved as 32-bit > PC-relative accesses. We could fix this with a fast large code model, but then > the kernel would need to relax global symbol references in modules and we don't > even do that for the simple code models we have now. FWIW, some of the > proposed large code models are essentially just split-PLT/GOT and therefor > don't require relaxation, but at that point we're essentially PIC until we > have more that 2GiB of kernel text -- and even then, we keep all the > performance issues. My memory might be out of date but I *think* we do it on powerpc without going to a large code model, but just having the in-kernel linker insert trampolines. Cheers, Ben.