Running qemu with KVM with two pts set will eventually trigger the following kernel warning: Jun 19 22:51:46 linux417 kernel: [ 10.863217] WARNING: CPU: 15 PID: 747 at /build/linux-jIx23a/linux-4.15.11/kernel/irq/chip.c:244 __irq_startup+0x5c/0x70 Jun 19 22:51:46 linux417 kernel: [ 10.863218] Modules linked in: xfs libcrc32c bochs_drm ttm joydev drm_kms_helper serio_raw ppdev parport_pc drm sg pcspkr evdev parport button ip_tables x _tables autofs4 ext4 crc16 mbcache jbd2 crc32c_generic fscrypto ecb crypto_simd cryptd glue_helper aes_x86_64 sd_mod ata_generic ata_piix libata psmouse e1000 scsi_mod i2c_piix4 floppy Jun 19 22:51:46 linux417 kernel: [ 10.863243] CPU: 15 PID: 747 Comm: (agetty) Not tainted 4.15.0-2-amd64 #1 Debian 4.15.11-1 Jun 19 22:51:46 linux417 kernel: [ 10.863244] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.1-0-g0551a4be2c-prebuilt.qemu-project.org 04/01/2014 Jun 19 22:51:46 linux417 kernel: [ 10.863247] RIP: 0010:__irq_startup+0x5c/0x70 Jun 19 22:51:46 linux417 kernel: [ 10.863248] RSP: 0018:ffffb05e41a7bb00 EFLAGS: 00010046 Jun 19 22:51:46 linux417 kernel: [ 10.863249] RAX: 0000000005030000 RBX: ffff8d6236811c00 RCX: 0000000000000004 Jun 19 22:51:46 linux417 kernel: [ 10.863250] RDX: ffff8d6236811c00 RSI: 0000000000000001 RDI: ffff8d6236811c98 Jun 19 22:51:46 linux417 kernel: [ 10.863251] RBP: 0000000000000004 R08: ffff8d6236400490 R09: ffff8d6236811c00 Jun 19 22:51:46 linux417 kernel: [ 10.863252] R10: 0000000000000000 R11: 0000000000000040 R12: 0000000000000001 Jun 19 22:51:46 linux417 kernel: [ 10.863253] R13: 0000000000000001 R14: 0000000000000000 R15: ffff8d622f063080 Jun 19 22:51:46 linux417 kernel: [ 10.863258] FS: 00007fb80ab02dc0(0000) GS:ffff8d62371c0000(0000) knlGS:0000000000000000 Jun 19 22:51:46 linux417 kernel: [ 10.863259] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 Jun 19 22:51:46 linux417 kernel: [ 10.863260] CR2: 00007fb80a4852c1 CR3: 00000002341c0000 CR4: 00000000000006e0 Jun 19 22:51:46 linux417 kernel: [ 10.863277] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 Jun 19 22:51:46 linux417 kernel: [ 10.863278] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Jun 19 22:51:46 linux417 kernel: [ 10.863278] Call Trace: Jun 19 22:51:46 linux417 kernel: [ 10.863287] irq_startup+0x57/0xe0 Jun 19 22:51:46 linux417 kernel: [ 10.863290] enable_irq+0x49/0x90 Jun 19 22:51:46 linux417 kernel: [ 10.863296] serial8250_do_startup+0x79d/0x810 Jun 19 22:51:46 linux417 kernel: [ 10.863298] uart_startup.part.22+0x5a/0x160 Jun 19 22:51:46 linux417 kernel: [ 10.863303] tty_port_open+0x89/0xd0 Jun 19 22:51:46 linux417 kernel: [ 10.863305] uart_open+0x3c/0x50 Jun 19 22:51:46 linux417 kernel: [ 10.863307] tty_open+0xf5/0x470 Jun 19 22:51:46 linux417 kernel: [ 10.863311] chrdev_open+0xc4/0x1d0 Jun 19 22:51:46 linux417 kernel: [ 10.863313] ? cdev_put.part.1+0x20/0x20 Jun 19 22:51:46 linux417 kernel: [ 10.863316] do_dentry_open+0x1b3/0x2f0 Jun 19 22:51:46 linux417 kernel: [ 10.863320] path_openat+0x611/0x1600 Jun 19 22:51:46 linux417 kernel: [ 10.863324] ? check_preempt_wakeup+0x102/0x230 Jun 19 22:51:46 linux417 kernel: [ 10.863327] do_filp_open+0x9b/0x110 Jun 19 22:51:46 linux417 kernel: [ 10.863330] ? tty_release_struct+0x2f/0x40 Jun 19 22:51:46 linux417 kernel: [ 10.863332] ? __check_object_size+0xaf/0x1b0 Jun 19 22:51:46 linux417 kernel: [ 10.863334] ? do_sys_open+0x1bd/0x250 Jun 19 22:51:46 linux417 kernel: [ 10.863335] do_sys_open+0x1bd/0x250 Jun 19 22:51:46 linux417 kernel: [ 10.863341] do_syscall_64+0x6e/0x130 Jun 19 22:51:46 linux417 kernel: [ 10.863347] entry_SYSCALL_64_after_hwframe+0x3d/0xa2 Jun 19 22:51:46 linux417 kernel: [ 10.863349] RIP: 0033:0x7fb80a6e83ab Jun 19 22:51:46 linux417 kernel: [ 10.863350] RSP: 002b:00007ffd76d3e310 EFLAGS: 00000246 ORIG_RAX: 0000000000000101 Jun 19 22:51:46 linux417 kernel: [ 10.863352] RAX: ffffffffffffffda RBX: 00005564a593f310 RCX: 00007fb80a6e83ab Jun 19 22:51:46 linux417 kernel: [ 10.863353] RDX: 0000000000080902 RSI: 00005564a5942970 RDI: 00000000ffffff9c Jun 19 22:51:46 linux417 kernel: [ 10.863354] RBP: 0000000000000015 R08: 00007ffd76d3e320 R09: 0000000000000000 Jun 19 22:51:46 linux417 kernel: [ 10.863355] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000080902 Jun 19 22:51:46 linux417 kernel: [ 10.863356] R13: 00005564a5942970 R14: 00005564a593f310 R15: 00005564a593ee70 Jun 19 22:51:46 linux417 kernel: [ 10.863357] Code: 92 00 48 8b 93 a8 00 00 00 81 22 ff ff fe ff 48 8b 93 a8 00 00 00 81 22 ff ff fd ff 48 8b 93 a8 00 00 00 81 0a 00 00 40 00 5b c3 <0f> 0b eb be 48 89 df e8 28 ff ff ff 31 c0 eb e1 0f 1f 40 00 0f Jun 19 22:51:46 linux417 kernel: [ 10.863383] ---[ end trace 92b708d2e2cee684 ]--- This is reproduced by running qemu with two pts, so something like: qemu-system-x86_64 -cpu kvm64 -enable-kvm -m 8192 -smp 4 \ -nographic -boot order=d -monitor pty -serial stdio \ -chardev pty,id=ttyS1 -device isa-serial,chardev=ttyS1 \ -chardev pty,id=ttyS2 -device isa-serial,chardev=ttyS2 \ Reproduced on qemu 2.12.0. Although the above triggers on v4.15 later kernels may also suffer it. The above was a debian v4.15 kernel. I'll test later when I have time with later kernels. After this warning we get this splat: Jun 19 22:51:46 linux417 kernel: [ 10.863800] spurious APIC interrupt through vector ff on CPU#0, should never happen. Jun 19 22:51:46 linux417 kernel: [ 10.868143] do_IRQ: 11.33 No irq handler for vector Jun 19 22:51:46 linux417 kernel: [ 10.889122] ------------[ cut here ]------------ Jun 19 22:51:46 linux417 kernel: [ 10.889124] Unbalanced enable for IRQ 4 Jun 19 22:51:46 linux417 kernel: [ 10.889144] WARNING: CPU: 15 PID: 747 at /build/linux-jIx23a/linux-4.15.11/kernel/irq/manage.c:526 __enable_irq+0x2f/0x60 Jun 19 22:51:46 linux417 kernel: [ 10.889145] Modules linked in: xfs libcrc32c bochs_drm ttm joydev drm_kms_helper serio_raw ppdev parport_pc drm sg pcspkr evdev parport button ip_tables x_tables autofs4 ext4 crc16 mbcache jbd2 crc32c_generic fscrypto ecb crypto_simd cryptd glue_helper aes_x86_64 sd_mod ata_generic ata_piix libata psmouse e1000 scsi_mod i2c_piix4 floppy Jun 19 22:51:46 linux417 kernel: [ 10.889161] CPU: 15 PID: 747 Comm: agetty Tainted: G W 4.15.0-2-amd64 #1 Debian 4.15.11-1 Jun 19 22:51:46 linux417 kernel: [ 10.889161] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.1-0-g0551a4be2c-prebuilt.qemu-project.org 04/01/2014 Jun 19 22:51:46 linux417 kernel: [ 10.889162] RIP: 0010:__enable_irq+0x2f/0x60 Jun 19 22:51:46 linux417 kernel: [ 10.889163] RSP: 0018:ffffb05e41a7bb30 EFLAGS: 00010082 Jun 19 22:51:46 linux417 kernel: [ 10.889165] RAX: 0000000000000000 RBX: ffff8d6236811c00 RCX: 0000000000000006 Jun 19 22:51:46 linux417 kernel: [ 10.889165] RDX: 0000000000000007 RSI: 0000000000000086 RDI: ffff8d62371d66f0 Jun 19 22:51:46 linux417 kernel: [ 10.889166] RBP: 0000000000000004 R08: 0000000000000273 R09: 0000000000000004 Jun 19 22:51:46 linux417 kernel: [ 10.889167] R10: 0000000000000000 R11: 0000000000000001 R12: 0000000000000246 Jun 19 22:51:46 linux417 kernel: [ 10.889167] R13: 0000000000000002 R14: 0000000000000000 R15: ffff8d622f063080 Jun 19 22:51:46 linux417 kernel: [ 10.889171] FS: 00007f1d1e5c0580(0000) GS:ffff8d62371c0000(0000) knlGS:0000000000000000 Jun 19 22:51:46 linux417 kernel: [ 10.889172] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 Jun 19 22:51:46 linux417 kernel: [ 10.889172] CR2: 00007f1d1e037260 CR3: 00000002332fe000 CR4: 00000000000006e0 Jun 19 22:51:46 linux417 kernel: [ 10.889185] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 Jun 19 22:51:46 linux417 kernel: [ 10.889186] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Jun 19 22:51:46 linux417 kernel: [ 10.889187] Call Trace: Jun 19 22:51:46 linux417 kernel: [ 10.889191] enable_irq+0x49/0x90 Jun 19 22:51:46 linux417 kernel: [ 10.889195] serial8250_do_startup+0x79d/0x810 Jun 19 22:51:46 linux417 kernel: [ 10.889196] uart_startup.part.22+0x5a/0x160 Jun 19 22:51:46 linux417 kernel: [ 10.889199] tty_port_open+0x89/0xd0 Jun 19 22:51:46 linux417 kernel: [ 10.889200] uart_open+0x3c/0x50 Jun 19 22:51:46 linux417 kernel: [ 10.889202] tty_open+0xf5/0x470 Jun 19 22:51:46 linux417 kernel: [ 10.889204] chrdev_open+0xc4/0x1d0 Jun 19 22:51:46 linux417 kernel: [ 10.889205] ? cdev_put.part.1+0x20/0x20 Jun 19 22:51:46 linux417 kernel: [ 10.889206] do_dentry_open+0x1b3/0x2f0 Jun 19 22:51:46 linux417 kernel: [ 10.889209] path_openat+0x611/0x1600 Jun 19 22:51:46 linux417 kernel: [ 10.889211] ? update_load_avg+0x563/0x6c0 Jun 19 22:51:46 linux417 kernel: [ 10.889212] ? update_load_avg+0x563/0x6c0 Jun 19 22:51:46 linux417 kernel: [ 10.889213] ? set_next_entity+0xa1/0x200 Jun 19 22:51:46 linux417 kernel: [ 10.889215] do_filp_open+0x9b/0x110 Jun 19 22:51:46 linux417 kernel: [ 10.889217] ? __check_object_size+0xaf/0x1b0 Jun 19 22:51:46 linux417 kernel: [ 10.889219] ? do_sys_open+0x1bd/0x250 Jun 19 22:51:46 linux417 kernel: [ 10.889220] do_sys_open+0x1bd/0x250 Jun 19 22:51:46 linux417 kernel: [ 10.889222] do_syscall_64+0x6e/0x130 Jun 19 22:51:46 linux417 kernel: [ 10.889225] entry_SYSCALL_64_after_hwframe+0x3d/0xa2 Jun 19 22:51:46 linux417 kernel: [ 10.889226] RIP: 0033:0x7f1d1e0d1c6e Jun 19 22:51:46 linux417 kernel: [ 10.889227] RSP: 002b:00007fff107ebd00 EFLAGS: 00000246 ORIG_RAX: 0000000000000101 Jun 19 22:51:46 linux417 kernel: [ 10.889229] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f1d1e0d1c6e Jun 19 22:51:46 linux417 kernel: [ 10.889229] RDX: 0000000000000902 RSI: 00007fff107ec6a0 RDI: 00000000ffffff9c Jun 19 22:51:46 linux417 kernel: [ 10.889230] RBP: 00007fff107ec220 R08: 00007fff107ebd10 R09: 00007fff107eff3b Jun 19 22:51:46 linux417 kernel: [ 10.889231] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000003 Jun 19 22:51:46 linux417 kernel: [ 10.889232] R13: 00007fff107eff3b R14: 00000000000002eb R15: 00007fff107ec6a0 Jun 19 22:51:46 linux417 kernel: [ 10.889233] Code: 00 8b 87 f0 00 00 00 85 c0 74 0e 83 f8 01 75 1e f6 87 ed 00 00 00 08 74 1f 8b b7 9c 00 00 00 48 c7 c7 48 b8 e1 ab e8 51 f9 f9 ff <0f> 0b c3 83 e8 01 89 87 f0 00 00 00 c3 81 8f e8 00 00 00 00 04 Jun 19 22:51:46 linux417 kernel: [ 10.889254] ---[ end trace 92b708d2e2cee685 ]--- And then we have blocking more than 120 seconds warnings: Jun 19 22:55:39 linux417 kernel: [ 242.652127] INFO: task kworker/27:2:351 blocked for more than 120 seconds. Jun 19 22:55:39 linux417 kernel: [ 242.661387] Tainted: G W 4.15.0-2-amd64 #1 Debian 4.15.11-1 Jun 19 22:55:39 linux417 kernel: [ 242.670626] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. Jun 19 22:55:39 linux417 kernel: [ 242.680951] kworker/27:2 D 0 351 2 0x80000000 Jun 19 22:55:39 linux417 kernel: [ 242.680963] Workqueue: events slab_caches_to_rcu_destroy_workfn Jun 19 22:55:39 linux417 kernel: [ 242.680964] Call Trace: Jun 19 22:55:39 linux417 kernel: [ 242.680974] ? __schedule+0x297/0x8b0 Jun 19 22:55:39 linux417 kernel: [ 242.680977] schedule+0x28/0x80 Jun 19 22:55:39 linux417 kernel: [ 242.680981] schedule_timeout+0x1e6/0x370 Jun 19 22:55:39 linux417 kernel: [ 242.680985] ? __slab_free+0x15a/0x300 Jun 19 22:55:39 linux417 kernel: [ 242.680987] ? __slab_free+0x15a/0x300 Jun 19 22:55:39 linux417 kernel: [ 242.680990] ? rcu_barrier_func+0x35/0x50 Jun 19 22:55:39 linux417 kernel: [ 242.680992] wait_for_completion+0x12c/0x190 Jun 19 22:55:39 linux417 kernel: [ 242.680997] ? wake_up_q+0x70/0x70 Jun 19 22:55:39 linux417 kernel: [ 242.680999] _rcu_barrier+0x124/0x180 Jun 19 22:55:39 linux417 kernel: [ 242.681002] slab_caches_to_rcu_destroy_workfn+0x8f/0xe0 Jun 19 22:55:39 linux417 kernel: [ 242.681006] process_one_work+0x17b/0x370 Jun 19 22:55:39 linux417 kernel: [ 242.681009] worker_thread+0x2e/0x390 Jun 19 22:55:39 linux417 kernel: [ 242.681011] ? process_one_work+0x370/0x370 Jun 19 22:55:39 linux417 kernel: [ 242.681169] kthread+0x113/0x130 Jun 19 22:55:39 linux417 kernel: [ 242.681175] ? kthread_create_worker_on_cpu+0x70/0x70 Jun 19 22:55:39 linux417 kernel: [ 242.681180] ret_from_fork+0x35/0x40 Jun 19 22:57:40 linux417 kernel: [ 363.484325] INFO: task kworker/27:2:351 blocked for more than 120 seconds. Jun 19 22:57:40 linux417 kernel: [ 363.494585] Tainted: G W 4.15.0-2-amd64 #1 Debian 4.15.11-1 Jun 19 22:57:40 linux417 kernel: [ 363.502931] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. Jun 19 22:57:40 linux417 kernel: [ 363.511982] kworker/27:2 D 0 351 2 0x80000000 Jun 19 22:57:40 linux417 kernel: [ 363.511993] Workqueue: events slab_caches_to_rcu_destroy_workfn Jun 19 22:57:40 linux417 kernel: [ 363.511995] Call Trace: Jun 19 22:57:40 linux417 kernel: [ 363.512003] ? __schedule+0x297/0x8b0 Jun 19 22:57:40 linux417 kernel: [ 363.512032] schedule+0x28/0x80 Jun 19 22:57:40 linux417 kernel: [ 363.512035] schedule_timeout+0x1e6/0x370 Jun 19 22:57:40 linux417 kernel: [ 363.512039] ? __slab_free+0x15a/0x300 Jun 19 22:57:40 linux417 kernel: [ 363.512040] ? __slab_free+0x15a/0x300 Jun 19 22:57:40 linux417 kernel: [ 363.512043] ? rcu_barrier_func+0x35/0x50 Jun 19 22:57:40 linux417 kernel: [ 363.512045] wait_for_completion+0x12c/0x190 Jun 19 22:57:40 linux417 kernel: [ 363.512049] ? wake_up_q+0x70/0x70 Jun 19 22:57:40 linux417 kernel: [ 363.512052] _rcu_barrier+0x124/0x180 Jun 19 22:57:40 linux417 kernel: [ 363.512054] slab_caches_to_rcu_destroy_workfn+0x8f/0xe0 Jun 19 22:57:40 linux417 kernel: [ 363.512058] process_one_work+0x17b/0x370 Jun 19 22:57:40 linux417 kernel: [ 363.512061] worker_thread+0x2e/0x390 Jun 19 22:57:40 linux417 kernel: [ 363.512062] ? process_one_work+0x370/0x370 Jun 19 22:57:40 linux417 kernel: [ 363.512064] kthread+0x113/0x130 Jun 19 22:57:40 linux417 kernel: [ 363.512066] ? kthread_create_worker_on_cpu+0x70/0x70 Jun 19 22:57:40 linux417 kernel: [ 363.512068] ret_from_fork+0x35/0x40