bhyve 11.1->stable/12 ZVOL-WinGuest slowdown – r358848 far beyond imagination

[Harry Schmalzbauer]

Hello,

after upgrading my ancient ivy-bridge (Xeon E3, no vAPIC CPU support) 
from 11.1 to stable/12 I noteced a quiet heavy slowdown of my windows 
guest (Server 2012R2).

The guest-disk is a ZVOL backed virtio-blk device, which I suspected 
suffers from performance regressions – especially since I use them in 
volmode=dev.
But then I found the guest spending almost one full vCPU cycles in 
interrupt handling during light SQL load; something I rarely see on 
windows.  No idea what caused the slowdown, but I remembered r358848 – 
which isn't MFCd yet, but I gave it a try.
Word's can't describe the surprise.  Never imagined any Win-Guest on 
bhyve can feel like that…
Thanks a lot to Sir Burmeister et. al.

Hope this will land in stable/12 soon.

Thanks,

-Harry