Background ========== I have a test environment which runs QEMU 4.2 with a plugin that runs two copies of a PCIE device simulator on a CentOS 7.5 host with an Ubuntu 18.04 guest. When running with a single QEMU CPU using: -cpu kvm64,+lahf_lm -M q35,kernel-irqchip=off -device intel-iommu,intremap=on Our tests run fine. But when running with multiple cpu?s: -cpu kvm64,+lahf_lm -M q35,kernel-irqchip=off -device intel-iommu,intremap=on -smp 2,sockets=1,cores=2 Some mmio reads to the simulated devices BAR 0 registers by our device driver running on the guest are returning are returning incorrect values. Running QEMU under gdb I see that the read request is reaching our simulated device correctly and that the correct result is being returned by the simulator. Using gdb I have tracked the return value all the way back up the call stack and the correct value is arriving in KVM_EXIT_MMIO in kvm_cpu_exec (qemu-4.2.0/accel/kvm/kvm-all.c:2365) but the value returned to the device driver which initiated the read is 0. Question ======== Is anyone else running QEMU 4.2 in multi cpu mode? Is anyone getting incorrect reads from memory mapped device registers when running in this mode? I would appreciate any pointers on how best to debug the flow from KVM_EXIT_MMIO back to the device driver running on the guest
