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Szmigiero wrote: > On 21.08.2025 10:18, Naveen N Rao wrote: >> On Tue, Aug 19, 2025 at 03:32:13PM +0200, Maciej S. Szmigiero wrote: >>> From: "Maciej S. Szmigiero" >>> >>> When AVIC is enabled the normal pre-VMRUN LAPIC TPR to VMCB::V_TPR >>> sync in >>> sync_lapic_to_cr8() is inhibited so any changed TPR in the LAPIC >>> state would >>> *not* get copied into the V_TPR field of VMCB. >>> >>> AVIC does sync between these two fields, however it does so only on >>> explicit guest writes to one of these fields, not on a bare VMRUN. >>> >>> This is especially true when it is the userspace setting LAPIC state via >>> KVM_SET_LAPIC ioctl() since userspace does not have access to the guest >>> VMCB. >> >> Dumb question: why is the VMM updating TPR? Is this related to live >> migration or such? > > In this case, VMM is resetting LAPIC state on machine reset. > >> I think I do see the problem described here, but when AVIC is >> temporarily inhibited. So, trying to understand if there are other flows >> involving the VMM where TPR could be updated outside of the guest. >> >>> >>> Practice shows that it is the V_TPR that is actually used by the AVIC to >>> decide whether to issue pending interrupts to the CPU (not TPR in >>> TASKPRI), >>> so any leftover value in V_TPR will cause serious interrupt delivery >>> issues >>> in the guest when AVIC is enabled. >>> >>> Fix this issue by explicitly copying LAPIC TPR to VMCB::V_TPR in >>> avic_apicv_post_state_restore(), which gets called from KVM_SET_LAPIC >>> and >>> similar code paths when AVIC is enabled. >>> >>> Add also a relevant set of tests to xapic_state_test so hopefully >>> we'll be protected against getting such regressions in the future. >> >> Do the new tests reproduce this issue? > > Yes, and also check quite a bit more of TPR-related functionality. > >>> >>> >>> Yes, this breaks real guests when AVIC is enabled. >>> Specifically, the one OS that sometimes needs different handling and its >>> name begins with letter 'W'. >> >> Indeed, Linux does not use TPR AFAIK. >> >> I believe it does, during the local APIC initialization. When Maciej determined the root cause of this issue, I was wondering why we have not seen it earlier in Linux. I found that Linux takes a defensive approach and drains all pending interrupts during lapic initialization. Essentially, for each CPU, Linux will: - temporarily disable the Local APIC (via Spurious Int Vector Reg) - set the TPR to accept all "regular" interrupts i.e. tpr=0x10 - drain all pending interrupts in ISR and/or IRR - attempt the above draining step a max of 512 times - then re-enable APIC and continue initialization The relevant code is in setup_local_APIC() https://elixir.bootlin.com/linux/v6.16/source/arch/x86/kernel/apic/apic.c#L1533-L1545 So without Maciej's proposed change, other OSs that are not as resilient could also be affected by this issue. Alejandro >> - Naveen >> > > Thanks, > Maciej > >