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Date: Fri, 18 Jul 2014 10:48:40 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] latest rc: virtio-blk hangs forever after migration
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To: =?UTF-8?B?TWFyY2luIEdpYnXFgmE=?= <m.gibula@beyond.pl>, Andrey Korolyov <andrey@xdel.ru>
Cc: Amit Shah <amit.shah@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>, Fam Zheng <famz@redhat.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>

Il 17/07/2014 15:25, Marcin Gibuła ha scritto:
> +static void do_kvm_cpu_synchronize_state_always(void *arg)
> +{
> +    CPUState *cpu = arg;
> +
> +    kvm_arch_get_registers(cpu);
> +}
> +

The name of the hack^Wfunction is tricky, because compared to 
do_kvm_cpu_synchronize_state there are three things you change:

1) you always synchronize the state

2) the next call to do_kvm_cpu_synchronize_state will do 
kvm_arch_get_registers

3) the next CPU entry will call kvm_arch_put_registers:

         if (cpu->kvm_vcpu_dirty) {
             kvm_arch_put_registers(cpu, KVM_PUT_RUNTIME_STATE);
             cpu->kvm_vcpu_dirty = false;
         }

It is easy to find out if the "fix" is related to 1 or 2/3: just write

      if (cpu->kvm_vcpu_dirty) {
          printf ("do_kvm_cpu_synchronize_state_always: look at 2/3\n");
          kvm_arch_get_registers(cpu);
      } else {
          printf ("do_kvm_cpu_synchronize_state_always: look at 1\n");
      }

To further refine between 2 and 3, I suppose you can set a breakpoint on 
cpu_synchronize_all_states and kvm_cpu_exec, and see which is called 
first after cpu_synchronize_all_states_always.

I still lean very much towards reverting the patches now.  We can 
reapply them, fixed, in 2.1.1.

Paolo