https://bugzilla.kernel.org/show_bug.cgi?id=217423 --- Comment #4 from robert.hoo.linux@xxxxxxxxx --- On 5/18/2023 8:48 PM, bugzilla-daemon@xxxxxxxxxx wrote: [...] (Sorry for late response) >> Can elaborate more on this hrtimer issue/code path? > > Below are the steps in detail, I traced them via bpftrace, to simplify the > analysis, the preemption timer on host is disabled, guest is running with > TSC timer deadline mode. > > TSC changes before save VM: > 1 create VM/VCPU, guest TSC start from 0 (VCPU initial value) > host_tsc0 = 0 + offset0 > 2 pause VM after guest start finished (about 200ms) > host_tsc1 = guest_tsc1 + offset0 > guest_tsc1_deadline = guest_tsc1 + expire1 > 3 save VM state > save guest_tsc1 by reading MSR_IA32_TSC > save guest_tsc1_deadline by reading MSR_IA32_TSC_DEADLINE > > TSC changes in restore VM (to simplify the analysis, step 4 > and step 5 ignore the host TSC changes in restore process): > 4 create VM/VCPU, guest TSC start from 0 (VCPU initial value) > host_tsc3 = 0 + offset1 > 5 restore VM state > set MSR_IA32_TSC by guest_tsc1 > set MSR_IA32_TSC_DEADLINE by guest_tsc1_deadline > 6 start VM > VCPU_RUN > > In step 5 setting MSR_IA32_TSC, because the guest_tsc1 is within 1 second, > KVM will take this update as TSC synchronize, then skip update offset1. > This means the guest TSC is still at 0 (initialize value). IIUC, here no matter synchronizing = true or false, offset will always be updated, i.e. __kvm_synchronize_tsc() will be called. But the offset value will differ. I guess your environment is tsc_stable, then offset = kvm->arch.cur_tsc_offset, which is 0. That is to say, the elapsed time isn't counted in by the heuristics method in current code, that's the culprit. static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data) { ... offset = kvm_compute_l1_tsc_offset(vcpu, data); ... /* * For a reliable TSC, we can match TSC offsets, and for an unstable * TSC, we add elapsed time in this computation. We could let the * compensation code attempt to catch up if we fall behind, but * it's better to try to match offsets from the beginning. */ if (synchronizing && vcpu->arch.virtual_tsc_khz == kvm->arch.last_tsc_khz) { if (!kvm_check_tsc_unstable()) { offset = kvm->arch.cur_tsc_offset; } else { u64 delta = nsec_to_cycles(vcpu, elapsed); data += delta; offset = kvm_compute_l1_tsc_offset(vcpu, data); } matched = true; } __kvm_synchronize_tsc(vcpu, offset, data, ns, matched); raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags); } >> An alternative, I think, is to bypass this directly write IA32_MSR_TSC way >> to set/sync TSC offsets, but follow new approach introduced in your VMM by >> >> commit 828ca89628bfcb1b8f27535025f69dd00eb55207 >> Author: Oliver Upton <oliver.upton@xxxxxxxxx> >> Date: Thu Sep 16 18:15:38 2021 +0000 >> >> KVM: x86: Expose TSC offset controls to userspace >> >> ... >> >> Documentation/virt/kvm/devices/vcpu.rst: >> >> 4.1 ATTRIBUTE: KVM_VCPU_TSC_OFFSET >> >> :Parameters: 64-bit unsigned TSC offset >> >> ... >> >> Specifies the guest's TSC offset relative to the host's TSC. The guest's >> TSC is then derived by the following equation: >> >> guest_tsc = host_tsc + KVM_VCPU_TSC_OFFSET >> >> The following describes a possible algorithm to use for this purpose >> ... > > "TSC counts the time during which the VM was paused.", This new feature works > for live migration. But if we save/restore VM with snapshot, the TSC should > be > paused either? > Not sure what's host's TSC situation when host is, say, suspended/hibernated. VM Save/Restore can refer to that. But, the key point of this new approach is to use OFFSET rather than direct TSC value, this is like x86 TSC_ADJUST was introduced, and is preferred. Via this new interface, "... Ensure that the KVM_CLOCK_REALTIME flag is set in the provided structure. KVM will advance the VM's kvmclock to account for elapsed time since recording the clock values.", therefore I think it can solve your problem, rather than modify the ancient and heuristics code at high risk. -- You may reply to this email to add a comment. You are receiving this mail because: You are watching the assignee of the bug.
