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a=VkNPw1HP01LnGYTKEx00:22 a=jiCTI4zE5U7BLdzWsZGv:22 a=RD47p0oAkeU5bO7t-o6f:22 a=VwQbUJbxAAAA:8 a=yPCof4ZbAAAA:8 a=cXhUbMmNm04wmYEBA-AA:9 a=QEXdDO2ut3YA:10 a=5yU3S35YU4bGjq-dph-N:22 a=Bho9c0fBagfJEIQBS7DQ:22 cc=ntf awl=host:12099 On Mon, Aug 17, 2026 1:15:02AM -0700, Marc Zyngier wrote: > On Sun, 16 Aug 2026 06:33:01 +0100, > Dongli Zhang wrote: > > [...] > >> 4. Guest kernel changes are not included. I may send it separately to keep >> this series limited to the KVM hypervisor. > > Hold on. Do you mean you are changing the guest visible behaviour of a > PV interface? That's an ABI. It *cannot* change unilaterally. No, this does not change the ABI. Taking x86 as an example, the Linux guest currently does not reset the stealtime accounting metadata, i.e. rq->prev_steal_time and rq->prev_steal_time_rq, when a vCPU is brought online. I meant to reset that metadata before enabling KVM x86 steal time via MSR_KVM_STEAL_TIME, so that the guest is not affected by a stale steal-time value from before the vCPU was offlined. For example, in the code below, the guest resets rq->prev_steal_time and rq->prev_steal_time_rq after enabling KVM steal time via MSR_KVM_STEAL_TIME. [PATCH 1/5] x86/kvm: Reset prev_steal_time and prev_steal_time_rq when enabling steal time https://lore.kernel.org/all/20260505003044.78693-2-dongli.zhang@oracle.com +void sched_steal_time_cpu_init(int cpu, u64 steal) +{ + struct rq *rq = cpu_rq(cpu); + + rq->prev_steal_time = steal; +#ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING + rq->prev_steal_time_rq = steal; +#endif +} + #ifdef CONFIG_HAVE_PV_STEAL_CLOCK_GEN static u64 native_steal_clock(int cpu) { @@ -337,6 +354,12 @@ static void kvm_register_steal_time(void) return; wrmsrq(MSR_KVM_STEAL_TIME, (slow_virt_to_phys(st) | KVM_MSR_ENABLED)); + + /* + * This CPU is not ready to be scheduled yet. + */ + sched_steal_time_cpu_init(cpu, kvm_steal_clock(cpu)); + pr_debug("stealtime: cpu %d, msr %llx\n", cpu, (unsigned long long) slow_virt_to_phys(st)); } > >> >> [PATCH 1/5] x86/kvm: Reset prev_steal_time and prev_steal_time_rq when enabling steal time >> https://urldefense.com/v3/__https://lore.kernel.org/all/20260505003044.78693-2-dongli.zhang@oracle.com__;!!ACWV5N9M2RV99hQ!Kt5J7E63dvSlZsI1Ovt02JMZuh0BwYZZupi0BMQXDxnYkQ6iPNwB7tuWgd7wSVOEVHCF40thrD8Apg$ >> >> 5. There is one remaining corner case: this series resets last_steal when >> the vCPU run PID changes, but not when steal time is enabled. If additional >> host run_delay is accumulated after the PID changes but before the guest >> enables steal time, that delta could be unexpectedly accounted to guest >> vCPU steal time. In practice, this should not happen for Linux guests. > > Why is Linux immune to this? Also, KVM does not cater for Linux guests > only. I should have explained this more clearly. I did not mean that Linux is immune in the architectural sense, or that KVM should rely on Linux guest behavior. The corner case I had in mind is: 1. The vCPU is run by a new host PID, so KVM resets last_steal to the new PID's current run_delay. 2. The guest has not enabled steal time yet. 3. The new host task accumulates additional run_delay before the guest enables steal time. 4. When steal time is later enabled, that pre-enable run_delay can be included in the first stealtime update. For the Linux guest on QEMU/KVM x86 path, this window should normally be very small. For example, on x86, a Linux guest enables KVM steal time immediately from the CPU online path. Also, QEMU does not enter KVM_RUN for an offline vCPU, so the normal QEMU/KVM hotplug path leaves little opportunity to accumulate run_delay before steal time is enabled. Thank you very much! Dongli Zhang