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X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_MED=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 9/25/25 1:44 AM, David Woodhouse wrote: > On Wed, 2025-09-24 at 13:53 -0700, Dongli Zhang wrote: >> >> >> On 9/23/25 10:47 AM, David Woodhouse wrote: >>> On Tue, 2025-09-23 at 10:25 -0700, Dongli Zhang wrote: >>>> >>>> >>>> On 9/23/25 9:26 AM, David Woodhouse wrote: >>>>> On Mon, 2025-09-22 at 12:37 -0700, Dongli Zhang wrote: >>>>>> On 9/22/25 11:16 AM, David Woodhouse wrote: >>>> >>>> [snip] >>>> >>>>>>> >>>>>>>> >>>>>>>> As demonstrated in my test, currently guest_tsc doesn't stop counting during >>>>>>>> blackout because of the lack of "MSR_IA32_TSC put" at >>>>>>>> kvmclock_vm_state_change(). Per my understanding, it is a bug and we may need to >>>>>>>> fix it. >>>>>>>> >>>>>>>> BTW, kvmclock_vm_state_change() already utilizes KVM_SET_CLOCK to re-configure >>>>>>>> kvm-clock before continuing the guest VM. >>>>> >>>>> Yeah, right now it's probably just introducing errors for a stop/start >>>>> of the VM. >>>> >>>> But that help can meet the expectation? >>>> >>>> Thanks to KVM_GET_CLOCK and KVM_SET_CLOCK, QEMU saves the clock with >>>> KVM_GET_CLOCK when the VM is stopped, and restores it with KVM_SET_CLOCK when >>>> the VM is continued. >>> >>> It saves the actual *value* of the clock. I would prefer to phrase that >>> as "it makes the clock jump backwards to the time at which the guest >>> was paused". >>> >>>> This ensures that the clock value itself does not change between stop and cont. >>>> >>>> However, QEMU does not adjust the TSC offset via MSR_IA32_TSC during stop. >>>> >>>> As a result, when execution resumes, the guest TSC suddenly jumps forward. >>> >>> Oh wow, that seems really broken. If we're going to make it experience >>> a time warp, we should at least be *consistent*. >>> >>> So a guest which uses the TSC for timekeeping should be mostly >>> unaffected by this and its wallclock should still be accurate. A guest >>> which uses the KVM clock will be hosed by it. >>> >>> I think we should fix this so that the KVM clock is unaffected too. >> >> From my understanding of your reply, the kvm-clock/tsc should always be adjusted >> whenever a QEMU VM is paused and then resumed (i.e. via stop/cont). > > I think I agree, except I still hate the way you use the word > 'adjusted'. > > If I look at my clock, and then go to sleep for a while and look at the > clock again, nobody *adjusts* it. It just keeps running. > > That's the effect we should always strive for, and that's how we should > think about it and talk about it. > > It's difficult to talk about clocks because what does it mean for a > clock to be "unchanged"? Does it mean that it should return the same > time value? Or that it should continue to count consistently? I would > argue that we should *always* use language which assumes the latter. > > Turning to physics for a clumsy analogy, it's about the frame of > reference. We're all on a moving train. I look at you in the seat > opposite me, I go to sleep for a while, and I wake up and you're still > there. Nobody has "adjusted" your position to accommodate for the > movement of the train while I was asleep. > Thank you very much for explanation! I will use something like "keeps running". > > > >> This applies to: >> >> - stop / cont >> - savevm / loadvm >> - live migration >> - cpr >> >> It is a bug if the clock jumps backwards to the time at which the guest was paused. >> >> The time elapsed while the VM is paused should always be accounted for and >> reflected in kvm-clock/tsc once the VM resumes. > > In particular, in *all* but the live migration case, there should be > basically nothing to do. No addition, no subtraction. Only restoring > the *existing* relationships, precisely as they were before. That is > the TSC *offset* value, and the precise TSC→kvmclock parameters, all > bitwise *exactly* the same as before. > > And the only thing that changes on live migration is that you have to > set the TSC offset such that the guest sees the values it *would* have > seen on the original host at any given moment in time... and doesn't > know it was kidnapped and moved onto a different train while it was > sleeping...? > I see. That means, only re-configure tsc_offset, while maintaining the tsc->kvmclock PVTI. That's the reason you would like to remove 'kvm_arch->kvmclock_offset' entirely as future work. Thank you very much! Dongli Zhang