We recently upgraded to 4.3.0 and have found that when changing disk QoS settings on VMs whilst IO-Threads is enabled causes them to segfault and the VM to reboot. We've been able to replicate this across several VMs. VMs with IO-Threads disabled/turned off do not segfault when changing the QoS. Mar 1 11:49:06 srvXX kernel: IO iothread1[30468]: segfault at fffffffffffffff8 ip 0000557649f2bd24 sp 00007f80de832f60 error 5 in qemu-kvm[5576498dd000+a03000] Mar 1 11:49:06 srvXX abrt-hook-ccpp: invalid number 'iothread1' Mar 1 11:49:11 srvXX libvirtd: 2019-03-01 00:49:11.116+0000: 13365: error : qemuMonitorIORead:609 : Unable to read from monitor: Connection reset by peer Happy to supply some more logs to someone if they'll help but just wondering whether anyone else has experienced this or knows of a current fix other than turning io-threads off. Cheers.