Re: [Qemu-devel] race between kvm-kmod-3.0 and kvm-kmod-3.3 // was: race condition in qemu-kvm-1.0.1

[Peter Lieven]

On 28.06.2012 11:39, Jan Kiszka wrote:
> On 2012-06-28 11:31, Peter Lieven wrote:
> > On 28.06.2012 11:21, Jan Kiszka wrote:
> > > On 2012-06-28 11:11, Peter Lieven wrote:
> > > > On 27.06.2012 18:54, Jan Kiszka wrote:
> > > > > On 2012-06-27 17:39, Peter Lieven wrote:
> > > > > > Hi all,
> > > > > >
> > > > > > i debugged this further and found out that kvm-kmod-3.0 is working with
> > > > > > qemu-kvm-1.0.1 while kvm-kmod-3.3 and kvm-kmod-3.4 are not. What is
> > > > > > working as well is kvm-kmod-3.4 with an old userspace (qemu-kvm-0.13.0).
> > > > > > Has anyone a clue which new KVM feature could cause this if a vcpu is in
> > > > > > an infinite loop?
> > > > > Before accusing kvm-kmod ;), can you check if the effect is visible with
> > > > > an original Linux 3.3.x or 3.4.x kernel as well?
> > > > sorry, i should have been more specific. maybe I also misunderstood sth.
> > > > I was believing that kvm-kmod-3.0 is basically what is in vanialla kernel
> > > > 3.0. If I use the ubuntu kernel from ubuntu oneiric (3.0.0) it works, if
> > > > I use
> > > > a self-compiled kvm-kmod-3.3/3.4 with that kernel it doesn't.
> > > > however, maybe we don't have to dig to deep - see below.
> > > kvm-kmod wraps and patches things to make the kvm code from 3.3/3.4
> > > working on an older kernel. This step may introduce bugs of its own.
> > > Therefore my suggestion to use a "real" 3.x kernel to exclude that risk
> > > first of all.
> > >
> > > > > Then, bisection the change in qemu-kvm that apparently resolved the
> > > > > issue would be interesting.
> > > > >
> > > > > If we have to dig deeper, tracing [1] the lockup would likely be helpful
> > > > > (all events of the qemu process, not just KVM related ones: trace-cmd
> > > > > record -e all qemu-system-x86_64 ...).
> > > > that here is bascially whats going on:
> > > >
> > > >     qemu-kvm-1.0-2506  [010] 60996.908000: kvm_mmio:             mmio read
> > > > len 3 gpa 0xa0000 val 0x10ff
> > > >       qemu-kvm-1.0-2506  [010] 60996.908000: vcpu_match_mmio:      gva
> > > > 0xa0000 gpa 0xa0000 Read GPA
> > > >       qemu-kvm-1.0-2506  [010] 60996.908000: kvm_mmio:             mmio
> > > > unsatisfied-read len 1 gpa 0xa0000 val 0x0
> > > >       qemu-kvm-1.0-2506  [010] 60996.908000: kvm_userspace_exit:   reason
> > > > KVM_EXIT_MMIO (6)
> > > >       qemu-kvm-1.0-2506  [010] 60996.908000: kvm_mmio:             mmio
> > > > read len 3 gpa 0xa0000 val 0x10ff
> > > >       qemu-kvm-1.0-2506  [010] 60996.908000: vcpu_match_mmio:      gva
> > > > 0xa0000 gpa 0xa0000 Read GPA
> > > >       qemu-kvm-1.0-2506  [010] 60996.908000: kvm_mmio:             mmio
> > > > unsatisfied-read len 1 gpa 0xa0000 val 0x0
> > > >       qemu-kvm-1.0-2506  [010] 60996.908000: kvm_userspace_exit:   reason
> > > > KVM_EXIT_MMIO (6)
> > > >       qemu-kvm-1.0-2506  [010] 60996.908000: kvm_mmio:             mmio
> > > > read len 3 gpa 0xa0000 val 0x10ff
> > > >       qemu-kvm-1.0-2506  [010] 60996.908000: vcpu_match_mmio:      gva
> > > > 0xa0000 gpa 0xa0000 Read GPA
> > > >       qemu-kvm-1.0-2506  [010] 60996.908000: kvm_mmio:             mmio
> > > > unsatisfied-read len 1 gpa 0xa0000 val 0x0
> > > >       qemu-kvm-1.0-2506  [010] 60996.908000: kvm_userspace_exit:   reason
> > > > KVM_EXIT_MMIO (6)
> > > >       qemu-kvm-1.0-2506  [010] 60996.908000: kvm_mmio:             mmio
> > > > read len 3 gpa 0xa0000 val 0x10ff
> > > >       qemu-kvm-1.0-2506  [010] 60996.908000: vcpu_match_mmio:      gva
> > > > 0xa0000 gpa 0xa0000 Read GPA
> > > >       qemu-kvm-1.0-2506  [010] 60996.908000: kvm_mmio:             mmio
> > > > unsatisfied-read len 1 gpa 0xa0000 val 0x0
> > > >       qemu-kvm-1.0-2506  [010] 60996.908000: kvm_userspace_exit:   reason
> > > > KVM_EXIT_MMIO (6)
> > > >
> > > > its doing that forever. this is tracing the kvm module. doing the
> > > > qemu-system-x86_64 trace is a bit compilcated, but
> > > > maybe this is already sufficient. otherwise i will of course gather this
> > > > info as well.
> > > That's only tracing KVM event, and it's tracing when things went wrong
> > > already. We may need a full trace (-e all) specifically for the period
> > > when this pattern above started.
> > i will do that. maybe i should explain that the vcpu is executing
> > garbage when this above starts. its basically booting from an empty
> > harddisk.
> >
> > if i understand correctly qemu-kvm loops in kvm_cpu_exec(CPUState *env);
> >
> > maybe the time to handle the monitor/qmp connection is just to short.
> > if i understand furhter correctly, it can only handle monitor connections
> > while qemu-kvm is executing kvm_vcpu_ioctl(env, KVM_RUN, 0); or am i
> > wrong here? the time spend in this state might be rather short.
> Unless you played with priorities and affinities, the Linux scheduler
> should provide the required time to the iothread.
I have a 1.1GB (85MB compressed) trace-file. If you have time to
look at it I could drop it somewhere.

We currently run all VMs with nice 1 because we observed that
this improves that controlability of the Node in case all VMs
have execessive CPU load. Running the VM unniced does
not change the behaviour unfortunately.

Peter
> > my concern is not that the machine hangs, just the the hypervisor is
> > unresponsive
> > and its impossible to reset or quit gracefully. the only way to get the
> > hypervisor
> > ended is via SIGKILL.
> Right. Even if the guest runs wild, you must be able to control the vm
> via the monitor etc. If not, that's a bug.
>
> Jan