[Qemu-devel] virtio-scsi-pci iothread spins at 100%

[Ed Swierk]

Recently I noticed that when I configure a virtio-scsi-pci device
using an iothread, as soon as the guest virtio-scsi driver loads, the
iothread spins at 100%:

  -object iothread,id=iothread1 -device virtio-scsi-pci,iothread=iothread1

This occurs whether or not a disk is attached, with either
poll-max-ns=0 or poll-max-ns=32768, and with Linux 3.13, 4.1 and 4.4
guests. The iothread stops spinning as soon as the guest driver is
unloaded.

I bisected the issue to commit 684e508c23d28af8d6ed2c62738a0f60447c8274:

  aio: add .io_poll_begin/end() callbacks

It doesn't seem to affect performance, but obviously consuming CPU
cycles when there's no disk attached is undesirable. Is this an
expected side effect of implementing iothread polling? Is there a way
to revert to the old non-polling behavior?

--Ed