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On 07-08-2026 16:12, Partha Satapathy wrote: > From: Partha Sarathi Satapathy > > Hello, > > We are investigating recurring guest hangs following vCPU hotplug on a KVM/QEMU > system. We would appreciate guidance on whether x86 CPU bring-up should reject > a hot-added CPU when its runtime LAPIC ID differs from its ACPI/firmware APIC > ID, especially when that runtime ID conflicts with an existing CPU. > > Hypervisor system: > > Kernel: 5.15.0-318.199.3.5.el9uek.x86_64 > CPU(s): 384 > On-line CPU(s) list: 0-383 > Vendor ID: AuthenticAMD > BIOS Vendor ID: Advanced Micro Devices, Inc. > Model name: AMD EPYC 9J25 96-Core Processor > NUMA node(s): 2 > NUMA node0 CPU(s): 0-95,192-287 > NUMA node1 CPU(s): 96-191,288-383 > > Guest system: > > Kernel: 5.15.0-318.199.3.5.el9uek.x86_64 > Machine type: QEMU Standard PC (Q35) > Guest CPU model: AMD EPYC 9J25 96-Core Processor > Guest topology: 2 NUMA nodes, 2 sockets, 95 cores/socket, 2 threads/core > Maximum vCPUs: 380 > > We observe this issue regularly on the same hypervisor platform. > The guest topology has maxcpus=380. A group of vCPUs is offlined and then > hot-added. > > For CPU 17, ACPI/firmware assigns APIC ID 0x011 on node 0. During hot-add, > however, the CPU reads its local APIC ID as 0x111: > > [64830.401824] smpboot: Booting Node 0 Processor 17 APIC 0x11 > [64830.402255] [Firmware Bug]: CPU17: APIC id mismatch. > Firmware: 11 APIC: 111 > > CPU bring-up then continues through start_secondary() and > set_cpu_sibling_map(). The runtime APIC ID causes CPU 17 to be decoded as > socket/die 1, core 8, thread 0. This collides with CPU 35, which has APIC ID > 0x111, and results in: > > sched: CPU #17's smt-sibling CPU #34 is not on the same node! > [node: 0 != 1]. Ignoring dependency. > > The CPU is allowed to proceed to the CPUHP online path despite both the APIC-ID > mismatch and invalid topology. > > Approximately 57 seconds later, the guest stalls globally: > > - migration/stopper threads wait in multi_cpu_stop() / > stop_machine_yield(); > - workqueues and RCU stall; > - CPU 0 has a pending call-single-data request for CPU 17; > - CPU 17 does not provide a valid crash-NMI register note; > - eventually panics the guest. > > We have observed multiple instances of the same issue. > > Reference dmesg : > [64830.401824] smpboot: Booting Node 0 Processor 17 APIC 0x11 > [64830.401985] kvm-clock: cpu 17, msr 1071a01441, secondary cpu clock > [64830.402255] [Firmware Bug]: CPU17: APIC id mismatch. Firmware: 11 APIC: 111 > [64830.404886] ------------[ cut here ]------------ > [64830.404887] sched: CPU #17's smt-sibling CPU #34 is not on the same node! [node: 0 != 1]. Ignoring dependency. > [64830.404899] WARNING: CPU: 17 PID: 0 at arch/x86/kernel/smpboot.c:434 topology_sane.isra.0+0x7f/0xa3 > [64830.404965] > [64830.404967] set_cpu_sibling_map+0x2cd/0x63a > [64830.404970] start_secondary+0x5b/0x152 > [64830.404971] secondary_startup_64_no_verify+0xc3/0x0 > [64830.404976] > [64830.404976] ---[ end trace 956bce4479c9bf6d ]--- > [64830.405334] kvm-guest: setup async PF for cpu 17 > [64830.405337] kvm-guest: stealtime: cpu 17, msr 1197a74080 > [64830.405470] Will online and init hotplugged CPU: 17 > [64830.651019] smpboot: Booting Node 1 Processor 35 APIC 0x111 > [64830.651133] kvm-clock: cpu 35, msr 1071a018c1, secondary cpu clock > [64887.589563] BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 56s! > > > Questions: > > 1. Is it intentional for x86 CPU hot-add to continue after this ACPI/runtime > APIC-ID mismatch? > > 2. Should CPU bring-up fail if the runtime APIC ID conflicts with an already > present CPU or creates invalid SMT/package/NUMA topology? > > 3. Is there an existing mainline validation path or known KVM/QEMU issue for > this case that we should use instead? > > We are separately investigating why QEMU/KVM presents the incorrect LAPIC ID. > That may be a hypervisor/firmware defect. The kernel robustness question is > whether Linux should permit CPU hot-add to continue when inconsistent firmware > and runtime APIC identities result in a conflicting logical CPU topology. > > The relevant UEK code path is identify_secondary_cpu(), including > validate_apic_and_package_id() in arch/x86/kernel/cpu/common.c, followed by > set_cpu_sibling_map() in arch/x86/kernel/smpboot.c. We would appreciate > guidance on the corresponding current upstream validation path and the > expected behavior for this condition. > > Thanks, > Partha