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]

does anyone know whats that here in handle_mmio?

    /* hack: Red Hat 7.1 generates these weird accesses. */
    if ((addr > 0xa0000-4 && addr <= 0xa0000) && kvm_run->mmio.len == 3)
        return 0;

thanks,
peter

On 28.06.2012 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.
>
> 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.
>
> thanks
> peter