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

[globalgorrilla at fastmail.fm]

Still the same with 4.5-rc4 and AUR qemu-git.

On 23 Jan 2016, at 18:55, Dan Ziemba wrote:

> 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
>
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