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[83.57.175.68]) by smtp.gmail.com with ESMTPSA id bw22sm3200150ejb.78.2021.02.25.07.43.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 25 Feb 2021 07:43:38 -0800 (PST) Subject: Re: QEMU Clock record and replay To: Arnabjyoti Kalita , qemu-discuss , qemu-devel References: From: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Message-ID: Date: Thu, 25 Feb 2021 16:43:36 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 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: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 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=-0.435, 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=unavailable 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: =?UTF-8?Q?Alex_Benn=c3=a9e?= , Pavel Dovgalyuk Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Cc'ing Pavel/Alex. On 2/25/21 4:09 PM, Arnabjyoti Kalita wrote: > Hello all, > > I am trying to understand how the clock values are recorded and replayed > in QEMU (when it runs in TCG mode). I have been specifically following > the document that has been provided here - > https://github.com/qemu/qemu/blob/master/docs/replay.txt > > > I have the following 4 questions to ask- > > - Why are clock values being replayed off a cache and not directly off > the file as seen here - > https://github.com/qemu/qemu/blob/stable-5.0/replay/replay-time.c#L45? > ? > > - I have a requirement to record and replay host clock values. Can it so > happen that if incorrect values of the host clock are replayed, then the > timers in the guest start expiring incorrectly, during replay? > > - Let's say we record the clock values when the guest starts in KVM > mode, and we replay the clock values when the guest starts in TCG mode. > Does the record-replay functionality change a lot? > > - Also, I couldn't understand the specific purpose of replaying the > 'virtual real time clock' ? Do I still need to record and replay the > 'virtual real time clock' if we record the clock values in KVM mode, and > replay the clock values in TCG mode? > > Thank you very much. > > Best Regards, > Arnab > >