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[83.51.215.31]) by smtp.gmail.com with ESMTPSA id dq10sm1875168ejc.85.2021.05.13.06.32.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 13 May 2021 06:32:45 -0700 (PDT) Subject: Re: RFC: Memory region accesses where .valid.min_access_size < .impl.min_access_size To: Jonathan Cameron References: <20210513124737.00002b2d@Huawei.com> <2d5fdb5f-36d3-c998-ee36-f9d66cea7120@redhat.com> <20210513140018.00004560@Huawei.com> From: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Message-ID: Date: Thu, 13 May 2021 15:32:44 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <20210513140018.00004560@Huawei.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=philmd@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=philmd@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.699, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=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 , =?UTF-8?Q?Alex_Benn=c3=a9e?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 5/13/21 3:00 PM, Jonathan Cameron wrote: > On Thu, 13 May 2021 14:36:27 +0200 > Philippe Mathieu-Daudé wrote: > >> On 5/13/21 2:23 PM, Peter Maydell wrote: >>> On Thu, 13 May 2021 at 12:49, Jonathan Cameron >>> wrote: >>>> 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 will 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 = 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? >> >> See also this patch from Francisco: >> https://www.mail-archive.com/qemu-devel@nongnu.org/msg636935.html >> >> 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 */ You might use the "MMIO test device" to write your tests, see: https://www.mail-archive.com/qemu-devel@nongnu.org/msg730716.html https://www.mail-archive.com/qemu-devel@nongnu.org/msg730720.html I have a branch tagged v2, need to rebase it and post... >>> 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