Comment # 7 on bug 1081431 from Matwey Kornilov

(In reply to Takashi Iwai from comment #6)
> Adding Joerg to Cc.
> 
> This might be the same cause we've been seeing in some bug reports with the
> nested KVM.

Long story in short for the reference. I've spent a lot of time to dig out the
root for the issue. Finally, I've accidentally found that 4.16-rc3 host kernel
has the issue fixed. From the user space point of view, the way to trigger the
issue is to use KVM_IOEVENTFD with len set to 0 (it is valid setting). When len
is set to non-zero value - everything works as expected. When it is set to
zero, then qemu is receiving notification storm from eventfd, and the guest
execution is frozen (only interruptions/timers are executed).