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

[Friedrich Oslage]

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.

I tried to bisect it, but apparently during the 4.3 merge window vfio
was completely broken, my VM won't even boot.

Can someone give me a hint on how to debug this? I am not really capable
of debugging the kernel, therefore bisecting was my only hope...

Regards,
Friedrich

On 01/17/2016 10:29 AM, Friedrich Oslage wrote:
> Hi,
>
> 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.
>
> When i start a simple "openssl speed" in the VM and use the task manager
> to pin it to a single cpu, it work's just the way I expect it to work: 1
> cpu has 100% usage, visible in both guest's task manager and host's top.
>
> But when I start a game, for instance Diablo 3 or CoD Black Ops III, and
> also use the task manager to pin it to a single cpu, the cpu usage shown
> in guest's task manager differs from the one shown in host's top: the
> guest shows 1 cpu with 100% usage, as expected, but the host shows all 6
> cpus with 100% usage.
>
> It only happens with games. If you use other multi-threaded programs
> without 3D, for instance prime95, the cpu usage in guest's task manager
> is equal to the one in top.
>
> I can only assume vfio is somehow causing this additional cpu usage, but
> a it seems to be.... to much.
>
> Did someone else experience this or can enlighten me on what qemu/vfio
> is doing with additional cpu time?
>
> Regards,
> Friedrich
>
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