[Bug 254513] virtio_random: random_harvestq spinning on a CPU with Q35 virtio random device

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254513

            Bug ID: 254513
           Summary: virtio_random: random_harvestq spinning on a CPU with
                    Q35 virtio random device
           Product: Base System
           Version: 13.0-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: misc
          Assignee: bugs at FreeBSD.org
          Reporter: me at igalic.co

This virtual machine is running @ Hetzner on their CPX series, which uses the
Qemu Q35 chipset. Support for this has been recently added to FreeBSD and is in
FreeBSD 13.0.

The problem manifests as a load average of 1 on a 2 CPU system. `top -S` shows:

  PID USERNAME    THR PRI NICE   SIZE    RES STATE    C   TIME    WCPU COMMAND
    8 root          1 -16    -     0B    16K   0B CPU0     0  10:42  99.95%
rand_harvestq


So lets dig into the random device.
The device is identified like this:

grep -i entropy /var/run/dmesg.boot
vtrnd0: <VirtIO Entropy Adapter> on virtio_pci4
random: registering fast source VirtIO Entropy Adapter

meena at websrv2-hel1 ~> devinfo -p vtrnd0 -v
vtrnd0 pnpinfo vendor=0x00001af4 device=0x1044 subvendor=0x1af4
device_type=0x00000004
 virtio_pci4 pnpinfo vendor=0x1af4 device=0x1044 subvendor=0x1af4
subdevice=0x1100 class=0x00ff00 at slot=0 function=0 dbsf=pci0:5:0:0
 pci5
 pcib5 pnpinfo vendor=0x1b36 device=0x000c subvendor=0x1b36 subdevice=0x0000
class=0x060400 at slot=2 function=4 dbsf=pci0:0:2:4                            
          pci0
 pcib0 pnpinfo _HID=PNP0A08 _UID=1 _CID=PNP0A03 at handle=\_SB_.PCI0
 acpi0
 nexus0
meena at websrv2-hel1 ~>


procstat kstack on rand_harvestq process looks like this:

    8 100097 rand_harvestq       -                   vtrnd_read random_kthread
fork_exit fork_trampoline 

suggesting this is virtio_related

After adding

devmatch_blacklist="virtio_random.ko"

to rc.conf, and rebooting, the CPU spinning issue goes away.

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