[vfio-users] cpu usage in guest != cpu usage in host, even with exclusive pinning

[Dan Ziemba]

On Tue, 2016-01-19 at 16:52 +0100, Friedrich Oslage wrote:
> I did some more testing and it turns out to be a kernel error,
> introduced somewhere around linux 4.3.
> 
> Short description of the error: 100% cpu usage while using 3d in a vm
> with vfio
> 
> My last known good kernel is 4.2.8, which gives me about 20% cpu
> usage
> while playing Diablo III. My first known bad kernel is 4.3.3, which
> gives me 100% cpu usage with every game, even while idle-ling in the
> menu.
> 
> On 01/17/2016 10:29 AM, Friedrich Oslage wrote:
> > my host system is linux-4.4.0 with qemu-2.5.0 and a 4-core i7.
> > Linux is
> > booted with isolcpus=1-3,5-7 to reserve 3 cores + threads for the
> > Windows 10 VM.
> > The VM's 3 cores(2 threads each) are pinned to the respective
> > physical
> > core/thread. The iothread is pinned to 1-3,5-7.

Interesting, I've also noticed similar behavior since upgrading to
kernel 4.4 and qemu 2.5.0.  I am running 4 cores / 8 threads for the VM
and pinned to individual host threads by libvirt, but I didn't boot
with the isolcpus arg. Emulator threads are pinned to the remaining 4
host threads.

Host is a i7-5930K, so 6 cores.  Guest is running Win 8.1.

Just sitting at the pause screen in Fallout: New Vegas, I have a single
CPU running at 100% according to both the windows task manager and the
host.  The host cpu thread that is at 100% is one of the ones with a
vcpu pinned to it.  In the guest, it does appear that the game is the
thing using all the CPU cycles, and the GPU is basically idle at this
time.

When I was previously running kernel 4.1.15 and qemu 2.4, New Vegas was
never demanding enough to max out any one cpu no matter what I was
doing with the game.  Same goes for more demanding games such as GTA V.
 

I upgraded both kernel and qemu at the same time, so I can't say which
caused it.  I haven't really noticed much difference in performance. If
anything, it might be slightly better.  I seemed to notice less frame
rate drops and stuttering while playing GTA V after the upgrade.

Dan