I'm unable to reproduce this issue. The host stays responsive and the dd command completes in a reasonable amount of time. QEMU does not exceed the 64-thread pool size.
Please post steps to reproduce the issue using a minimal command-line without libvirt. Here is information on my attempt to reproduce the problem: Guest: Kernel 4.10.8-200.fc25.x86_64 Host: 4.10.11-200.fc25.x86_64 QEMU: qemu.git/master (e619b14746e5d8c0e53061661fd0e1da01fd4d60) The LV is 1 GB on top of LUKS on a Samsung MZNLN256HCHP SATA SSD drive. mpstat -P ALL 5 output: 11:02:02 AM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle 11:02:07 AM all 3.36 0.00 6.22 34.54 0.25 0.50 0.00 3.11 0.00 52.03 11:02:07 AM 0 2.82 0.00 5.63 32.39 0.80 1.21 0.00 3.22 0.00 53.92 11:02:07 AM 1 3.02 0.00 6.04 28.77 0.20 0.20 0.00 3.02 0.00 58.75 11:02:07 AM 2 3.56 0.00 7.71 44.27 0.20 0.40 0.00 2.37 0.00 41.50 11:02:07 AM 3 3.81 0.00 5.61 32.46 0.00 0.40 0.00 4.01 0.00 53.71 vmstat 5 output: procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu----- r b swpd free buff cache si so bi bo in cs us sy id wa st 0 0 0 1617404 6484 3541468 0 0 2145 84794 1976 8814 8 8 64 20 0 0 0 0 1619492 6484 3538592 0 0 613 69340 1518 7430 6 7 70 17 0 0 0 0 1618920 6484 3538680 0 0 280 75199 1421 6811 6 7 52 35 0 pidstat -v -p $PID_OF_QEMU 5 output: 11:01:08 AM UID PID threads fd-nr Command 11:02:03 AM 0 13043 67 37 qemu-system-x86 11:02:08 AM 0 13043 67 37 qemu-system-x86 11:02:13 AM 0 13043 67 37 qemu-system-x86 $ sudo x86_64-softmmu/qemu-system-x86_64 -enable-kvm -m 1024 -cpu host \ -device virtio-scsi-pci,id=scsihw0,bus=pci.0,addr=0x5 \ -drive file=test.img,if=none,id=drive-scsi0,format=raw,cache=none,aio=native,detect-zeroes=on \ -device scsi-hd,bus=scsihw0.0,channel=0,scsi-id=0,lun=0,drive=drive-scsi0,id=scsi0,bootindex=100 \ -drive file=/dev/path/to/testlv,if=none,id=drive-scsi1,format=raw,cache=none,aio=native,detect-zeroes=on \ -device scsi-hd,bus=scsihw0.0,channel=0,scsi-id=0,lun=1,drive=drive-scsi1,id=scsi1,bootindex=101 \ -nographic guest# dd if=/dev/zero of=/dev/sdb bs=1G count=1 oflag=direct 1+0 records in 1+0 records out 1073741824 bytes (1.1 GB, 1.0 GiB) copied, 15.0681 s, 71.3 MB/s -- You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/1687653 Title: QEMU-KVM / detect_zeroes causes KVM to start unlimited number of threads on Guest-Sided High-IO with big Blocksize Status in QEMU: Confirmed Status in Ubuntu: Confirmed Bug description: QEMU-KVM in combination with "detect_zeroes=on" makes a Guest able to DoS the Host. This is possible if the Host itself has "detect_zeroes" enabled and the Guest writes a large Chunk of data with a huge blocksize onto the drive. E.g.: dd if=/dev/zero of=/tmp/DoS bs=1G count=1 oflag=direct All QEMU-Versions after implementation of detect_zeroes are affected. Prior are unaffected. This is absolutely critical, please fix this ASAP! ##### Provided by Dominik Csapak: source , bs , count , O_DIRECT, behaviour urandom , bs 1M, count 1024, O_DIRECT: OK file , bs 1M, count 1024, O_DIRECT: OK /dev/zero , bs 1M, count 1024, O_DIRECT: OK zero file , bs 1M, count 1024, O_DIRECT: OK /dev/zero , bs 1G, count 1, O_DIRECT: NOT OK zero file , bs 1G, count 1, O_DIRECT: NOT OK zero file , bs 1G, count 1, no O_DIRECT: NOT OK rand file , bs 1G, count 1, O_DIRECT: OK rand file , bs 1G, count 1, no O_DIRECT: OK discard on: urandom , bs 1M, count 1024, O_DIRECT: OK rand file , bs 1M, count 1024, O_DIRECT: OK /dev/zero , bs 1M, count 1024, O_DIRECT: OK zero file , bs 1M, count 1024, O_DIRECT: OK /dev/zero , bs 1G, count 1, O_DIRECT: NOT OK zero file , bs 1G, count 1, O_DIRECT: NOT OK zero file , bs 1G, count 1, no O_DIRECT: NOT OK rand file , bs 1G, count 1, O_DIRECT: OK rand file , bs 1G, count 1, no O_DIRECT: OK detect_zeros off: urandom , bs 1M, count 1024, O_DIRECT: OK rand file , bs 1M, count 1024, O_DIRECT: OK /dev/zero , bs 1M, count 1024, O_DIRECT: OK zero file , bs 1M, count 1024, O_DIRECT: OK /dev/zero , bs 1G, count 1, O_DIRECT: OK zero file , bs 1G, count 1, O_DIRECT: OK zero file , bs 1G, count 1, no O_DIRECT: OK rand file , bs 1G, count 1, O_DIRECT: OK rand file , bs 1G, count 1, no O_DIRECT: OK ##### Provided by Florian Strankowski bs - count - io-threads 512K - 2048 - 2 1M - 1024 - 2 2M - 512 - 4 4M - 256 - 6 8M - 128 - 10 16M - 64 - 18 32M - 32 - uncountable Please refer to further information here: https://bugzilla.proxmox.com/show_bug.cgi?id=1368 To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/1687653/+subscriptions