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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id y7sm1863687wmg.40.2020.10.21.00.37.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 21 Oct 2020 00:37:54 -0700 (PDT) Subject: Re: [PATCH 1/1] Skip flatview_simplify() for cpu vendor zhaoxin To: Alex Williamson References: <20201016112933.14856-1-FelixCui-oc@zhaoxin.com> <20201016112933.14856-2-FelixCui-oc@zhaoxin.com> <20201019130206.1d3baffc@w520.home> <4d2b96cb-8f7f-2598-39e6-4cf0f61d567b@redhat.com> <20201020164437.5e3cb4ce@w520.home> From: Paolo Bonzini Message-ID: <783b5ef0-277c-363d-f342-7c0351f9ac16@redhat.com> Date: Wed, 21 Oct 2020 09:37:53 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.1 MIME-Version: 1.0 In-Reply-To: <20201020164437.5e3cb4ce@w520.home> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=pbonzini@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=63.128.21.124; envelope-from=pbonzini@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/10/20 22:12:28 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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_NONE=-0.0001, RCVD_IN_MSPIKE_H5=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: FelixCuioc , Eduardo Habkost , CobeChen-oc@zhaoxin.com, qemu-devel@nongnu.org, TonyWWang-oc@zhaoxin.com, RockCui-oc@zhaoxin.com, Richard Henderson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 21/10/20 00:44, Alex Williamson wrote: > Do we necessarily need a memory map ioctl for this or could it be the > QEMU code that compares the old and new maps to trigger map and unmap > ioctls? For example (aiui) our race is that if we have contiguous > memory regions A and B and flatview_simplify() tries to expand A and > delete B we'll see a series of listener notifications deleting A and B > and adding A'. But the vfio QEMU code could parse the memory map to > determine that old A + B is functionally equivalent to A' and do > nothing. I think the issue is a bit different, and in fact there are two sides of the same issue. Say you have A (large) and it is replaced by A' (smaller) + B, then: * the first part of A disappears for a moment before A' appears. This is something that QEMU can work around, by not doing anything * the second part of A disappears for a moment before B appears. This is the root API issue and not something that QEMU can work around; and in fact it is not even fixed by removing flatview_simplify. Felix, did you identify the regions whose simplification causes the bug? Is this RAM (for example due to the PAM registers) or something else? Paolo