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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id y6sm32981201wrn.41.2020.09.16.04.59.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 16 Sep 2020 04:59:44 -0700 (PDT) From: Vitaly Kuznetsov To: "Dr. David Alan Gilbert" , Antoine Damhet Subject: Re: [BUG] Migration hv_time rollback In-Reply-To: <20200916112956.GE2833@work-vm> References: <20200916090602.blkm7eym6g5bnvvk@tartarus> <20200916112956.GE2833@work-vm> Date: Wed, 16 Sep 2020 13:59:43 +0200 Message-ID: <87bli6hqz4.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=vkuznets@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=216.205.24.124; envelope-from=vkuznets@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/09/16 02:16:02 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -50 X-Spam_score: -5.1 X-Spam_bar: ----- X-Spam_report: (-5.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-2.999, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, 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: Juan Quintela , Marcelo Tosatti , qemu-devel@nongnu.org, "Michael S. Tsirkin" , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" "Dr. David Alan Gilbert" writes: > cc'ing in Vitaly who knows about the hv stuff. > cc'ing Marcelo who knows about clocksources :-) > * Antoine Damhet (antoine.damhet@blade-group.com) wrote: >> Hi, >> >> We are experiencing timestamp rollbacks during live-migration of >> Windows 10 guests Are you migrating to the same hardware (with the same TSC frequency)? Is TSC used as the clocksource on the host? >> with the following qemu configuration (linux 5.4.46 >> and qemu master): >> ``` >> $ qemu-system-x86_64 -enable-kvm -cpu host,kvm=off,hv_time [...] >> ``` Out of pure curiosity, what's the purpose of doing 'kvm=off'? Windows is not going to check for KVM identification anyway so we pretend we're Hyper-V. Also, have you tried adding more Hyper-V enlightenments? > > How big a jump are you seeing, and how did you notice it in the guest? > > Dave > >> I have tracked the bug to the fact that `kvmclock` is not exposed and >> disabled from qemu PoV but is in fact used by `hv-time` (in KVM). >> >> I think we should enable the `kvmclock` (qemu device) if `hv-time` is >> present and add Hyper-V support for the `kvmclock_current_nsec` >> function. AFAICT kvmclock_current_nsec() checks whether kvmclock was enabled by the guest: if (!(env->system_time_msr & 1ULL)) { /* KVM clock not active */ return 0; } and this is (and way) always false for Windows guests. >> >> I'm asking for advice because I am unsure this is the _right_ approach >> and how to keep migration compatibility between qemu versions. >> >> Thank you all, >> >> -- >> Antoine 'xdbob' Damhet -- Vitaly