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[83.59.163.107]) by smtp.gmail.com with ESMTPSA id lo26sm1099833ejb.106.2021.02.03.07.02.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 03 Feb 2021 07:02:21 -0800 (PST) Subject: Re: ARM Snapshots Not Backwards-Compatible To: Aaron Lindsay , Peter Maydell References: From: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Message-ID: <2572efa4-8aa3-32e4-7559-f93e6522d284@redhat.com> Date: Wed, 3 Feb 2021 16:02:19 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: 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: 7bit 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=-0.539, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.178, RCVD_IN_DNSWL_LOW=-0.7, 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: Andrew Jones , qemu-arm , qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , Juan Quintela Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 2/3/21 3:58 PM, Aaron Lindsay wrote: > On Feb 03 10:01, Peter Maydell wrote: >>> The third is that meanings of the bits in env->features (as defined by >>> `enum arm_features` in target/arm/cpu.h) has shifted. For example, >>> ARM_FEATURE_PXN, ARM_FEATURE_CRC, ARM_FEATURE_VFP, ARM_FEATURE_VFP3, >>> ARM_FEATURE_VFP4 have all been removed and ARM_FEATURE_V8_1M has been >>> added since 4.1.0. Heck, even I have added a field there in the past. >>> Unfortunately, these additions/removals mean that when env->features is >>> saved on one version and restored on another the bits can mean different >>> things. Notably, the removal of the *VFP features means that a snapshot >>> of a CPU reporting it supports ARM_FEATURE_VFP3 on 4.1.0 thinks it's now >>> ARM_FEATURE_M on 5.2.0! >> >> Ow. I didn't realize the env->features was in the migration state :-( >> There is no reason for it to be, because it's a constant property >> of the CPU. The easy fix is to replace >> VMSTATE_UINT64(env.features, ARMCPU), >> in target/arm/machine.c with whatever the syntax is for "ignore >> 64 bits of data here". Then we'll ignore whatever is coming in >> from the source, which we don't need, and we'll stop sending it >> out if we're the destination. > > I'll look into this. I think this is: VMSTATE_UNUSED(sizeof(uint64_t)) > > -Aaron >