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If a user did: echo 0 > /sys/devices/system/cpu/cpu1/online echo 1 > /sys/devices/system/cpu/cpu1/online we would (this a build with DEBUG enabled) get to: smpboot: ++++++++++++++++++++=_---CPU UP 1 .. snip.. smpboot: Stack at about ffff880074c0ff44 smpboot: CPU1: has booted. and hang. The RCU mechanism would kick in an try to IPI the CPU1 but the IPIs (and all other interrupts) would never arrive at the CPU1. At first glance at least. A bit digging in the hypervisor trace shows that (using xenanalyze): [vla] d4v1 vec 243 injecting 0.043163027 --|x d4v1 intr_window vec 243 src 5(vector) intr f3 ] 0.043163639 --|x d4v1 vmentry cycles 1468 ] 0.043164913 --|x d4v1 vmexit exit_reason PENDING_INTERRUPT eip ffffffff81673254 0.043164913 --|x d4v1 inj_virq vec 243 real [vla] d4v1 vec 243 injecting 0.043164913 --|x d4v1 intr_window vec 243 src 5(vector) intr f3 ] 0.043165526 --|x d4v1 vmentry cycles 1472 ] 0.043166800 --|x d4v1 vmexit exit_reason PENDING_INTERRUPT eip ffffffff81673254 0.043166800 --|x d4v1 inj_virq vec 243 real [vla] d4v1 vec 243 injecting there is a pending event (subsequent debugging shows it is the IPI from the VCPU0 when smpboot.c on VCPU1 has done "set_cpu_online(smp_processor_id(), true)") and the guest VCPU1 is interrupted with the callback IPI (0xf3 aka 243) which ends up calling __xen_evtchn_do_upcall. The __xen_evtchn_do_upcall seems to do *something* but not acknowledge the pending events. And the moment the guest does a 'cli' (that is the ffffffff81673254 in the log above) the hypervisor is invoked again to inject the IPI (0xf3) to tell the guest it has pending interrupts. This repeats itself forever. The culprit was the per_cpu(xen_vcpu, cpu) pointer. At the bootup we set each per_cpu(xen_vcpu, cpu) to point to the shared_info->vcpu_info[vcpu] but later on use the VCPUOP_register_vcpu_info to register per-CPU structures (xen_vcpu_setup). This is used to allow events for more than 32 VCPUs and for performance optimizations reasons. When the user performs the VCPU hotplug we end up calling the the xen_vcpu_setup once more. We make the hypercall which returns -EINVAL as it does not allow multiple registration calls (and already has re-assigned where the events are being set). We pick the fallback case and set per_cpu(xen_vcpu, cpu) to point to the shared_info->vcpu_info[vcpu] (which is a good fallback during bootup). However the hypervisor is still setting events in the register per-cpu structure (per_cpu(xen_vcpu_info, cpu)). As such when the events are set by the hypervisor (such as timer one), and when we iterate in __xen_evtchn_do_upcall we end up reading stale events from the shared_info->vcpu_info[vcpu] instead of the per_cpu(xen_vcpu_info, cpu) structures. Hence we never acknowledge the events that the hypervisor has set and the hypervisor keeps on reminding us to ack the events which we never do. The fix is simple. Don't on the second time when xen_vcpu_setup is called over-write the per_cpu(xen_vcpu, cpu) if it points to per_cpu(xen_vcpu_info). Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com> CC: stable@vger.kernel.org Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> |
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| .. | ||
| boot | ||
| configs | ||
| crypto | ||
| ia32 | ||
| include | ||
| kernel | ||
| kvm | ||
| lguest | ||
| lib | ||
| math-emu | ||
| mm | ||
| net | ||
| oprofile | ||
| pci | ||
| platform | ||
| power | ||
| realmode | ||
| syscalls | ||
| tools | ||
| um | ||
| vdso | ||
| video | ||
| xen | ||
| .gitignore | ||
| Kbuild | ||
| Kconfig | ||
| Kconfig.cpu | ||
| Kconfig.debug | ||
| Makefile | ||
| Makefile_32.cpu | ||
| Makefile.um | ||