Re: QEMU KVM - Windows Server guest

["Radim Krčmář" <rkrcmar@redhat.com>]

2017-04-13 15:05+0100, Stefan Hajnoczi:
> On Wed, Apr 12, 2017 at 09:11:12PM +0200, Jean Baptiste Guerraz wrote:
> > Hello,
> > 
> > We're facing an issue with a Windows guest VM which runs quite well on
> > a laptop (Fedora 25 - Core I7 4720HQ -
> > https://www.asus.com/Notebooks/N551JX/specifications/ ) but not on 2
> > different servers (one Debian 8 on
> > https://documentation.online.net/en/dedicated-server/offers/2015/server-dedibox-pro-2015-gen2#server_dedibox_pro_2015_gen2
> > - and one Fedora 25 on
> > https://documentation.online.net/en/dedicated-server/offers/2016/server-dedibox-md-2016#server_dedibox_md_2016)

Interesting, is there some consistent difference between

  grep . /sys/module/kvm*/parameters/*

on those systems?

> > qemu-system-x86  7420 [000] 28829.151599: kvm:kvm_mmio: mmio read len
> > 4 gpa 0xfed000f0 val 0x5e3c5955
> 
> This is the physical address range of the HPET (timer).

Right, disabling HPET is worth a shot. :)

> > If one of you have an idea about how to dig further, that would be super :)
> 
> I looked at a few of the interrupts that were injected.  An interrupt is
> interrupt delivered every 15 milliseconds.  They were immediately
> acknowledged by the interrupt handler function inside the guest.
> 
> This just looks like a running guest that's doing no I/O to me.

It had two vector 47 interrupts earlier and those only got delivered
after TPR was lowered, so the problem could a bug in KVM's TPR handling,
which only allows vector 209 after some point?

> Can anyone else spot something suspicious that indicates 100% guest CPU
> consumption?

It seems to be reading HPET in a tight loop.  No idea why, though.

So far, I'd compare kvm parameters, disable HPET, and check TPR, to see
where that goes,

thanks.