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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 59580C433ED for ; Thu, 13 May 2021 13:18:08 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (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 B6A3761435 for ; Thu, 13 May 2021 13:18:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B6A3761435 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=Huawei.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:59822 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lhBE6-0008UC-F9 for qemu-devel@archiver.kernel.org; Thu, 13 May 2021 09:18:06 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:39718) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lhAyl-00008y-Fa for qemu-devel@nongnu.org; Thu, 13 May 2021 09:02:16 -0400 Received: from frasgout.his.huawei.com ([185.176.79.56]:2133) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lhAyi-0008Nh-4R for qemu-devel@nongnu.org; Thu, 13 May 2021 09:02:15 -0400 Received: from fraeml714-chm.china.huawei.com (unknown [172.18.147.201]) by frasgout.his.huawei.com (SkyGuard) with ESMTP id 4Fgs2m1gKhz70gTr; Thu, 13 May 2021 20:50:40 +0800 (CST) Received: from lhreml710-chm.china.huawei.com (10.201.108.61) by fraeml714-chm.china.huawei.com (10.206.15.33) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Thu, 13 May 2021 15:02:02 +0200 Received: from localhost (10.52.125.147) by lhreml710-chm.china.huawei.com (10.201.108.61) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Thu, 13 May 2021 14:02:01 +0100 Date: Thu, 13 May 2021 14:00:18 +0100 From: Jonathan Cameron To: Philippe =?ISO-8859-1?Q?Mathieu-Daud=E9?= Subject: Re: RFC: Memory region accesses where .valid.min_access_size < .impl.min_access_size Message-ID: <20210513140018.00004560@Huawei.com> In-Reply-To: <2d5fdb5f-36d3-c998-ee36-f9d66cea7120@redhat.com> References: <20210513124737.00002b2d@Huawei.com> <2d5fdb5f-36d3-c998-ee36-f9d66cea7120@redhat.com> Organization: Huawei Technologies Research and Development (UK) Ltd. X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; i686-w64-mingw32) MIME-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: quoted-printable X-Originating-IP: [10.52.125.147] X-ClientProxiedBy: lhreml753-chm.china.huawei.com (10.201.108.203) To lhreml710-chm.china.huawei.com (10.201.108.61) X-CFilter-Loop: Reflected Received-SPF: pass client-ip=185.176.79.56; envelope-from=jonathan.cameron@huawei.com; helo=frasgout.his.huawei.com X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Peter Maydell , Ben Widawsky , David Hildenbrand , Andrew Jeffery , Francisco Iglesias , "Michael S. Tsirkin" , Chris Browy , Linuxarm , Peter Xu , QEMU Developers , Paolo Bonzini , Igor Mammedov , Dan Williams , Alex =?ISO-8859-1?Q?Benn=E9e?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, 13 May 2021 14:36:27 +0200 Philippe Mathieu-Daud=E9 wrote: > On 5/13/21 2:23 PM, Peter Maydell wrote: > > On Thu, 13 May 2021 at 12:49, Jonathan Cameron > > wrote: =20 > >> My initial suggestion was to fix this by adding the relatively > >> simple code needed in the driver to implement byte read / write, > >> but Ben pointed at the QEMU docs - docs/devel/memory.rst which > >> says > >> " > >> .impl.min_access_size, .impl.max_access_size define the access sizes > >> (in bytes) supported by the *implementation*; other access sizes wi= ll be > >> emulated using the ones available. For example a 4-byte write will = be > >> emulated using four 1-byte writes, if .impl.max_access_size =3D 1. > >> " > >> > >> This isn't true when we have the situation where > >> .valid.min_access_size < .imp.min_access_size > >> > >> So change the docs or try to make this work? =20 >=20 > See also this patch from Francisco: > https://www.mail-archive.com/qemu-devel@nongnu.org/msg636935.html >=20 > And full unaligned access support from Andrew: > https://www.mail-archive.com/qemu-devel@nongnu.org/msg461247.html Thanks - that's very similar to what I was carrying, but I think it only covers the read case. That's backed up by the comment: /* XXX: Can't do this hack for writes */ >=20 > > I don't (yet) have a view on what the in-principle right thing > > should be, but in practice: how many devices do we have which > > set .valid.min_access_size < .imp.min_access_size ? If we want > > to change the semantics we'd need to look at those to see if they > > need to be adjusted (or if they're just currently buggy and would > > be fixed by the change). I'm only aware of this one CXL emulated device (+ the proposed code in the ADC in the above patch set). For the CXL device, working around this limitation is straight forward if that's the right option + updating the docs to slightly reduced chances of this being hit in the future. Thanks, Jonathan > >=20 > > thanks > > -- PMM > > =20 >=20