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Date: Fri, 17 Apr 2009 10:59:11 -0300
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From: Glauber Costa <glommer@gmail.com>
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Subject: [Qemu-devel] Re: [PATCH] return default values for apic probe
	functions.
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To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: qemu-devel@nongnu.org, Glauber Costa <glommer@redhat.com>, avi@redhat.com, kvm@vger.kernel.org, aliguori@us.ibm.com

On Fri, Apr 17, 2009 at 10:53 AM, Marcelo Tosatti <mtosatti@redhat.com> wro=
te:
> Hi Glauber,
>
> On Fri, Apr 17, 2009 at 01:15:21AM -0400, Glauber Costa wrote:
>> As KVM cpus runs on threads, it is possible that
>> we call kvm_load_registers() from a cpu thread, while the
>> apic has not yet fully initialized. kvm_load_registers() is called
>> from ap_main_loop.
>>
>> This is not a problem when we're starting the whole machine together,
>> but is a problem for hotplug, since we don't have the protection
>> of the locks that protect machine initialization. Currently, some execut=
ions
>> of cpu hotplug on rainy sundays fail with a segfault.
>
> =C2=A0 =C2=A0/* and wait for machine initialization */
> =C2=A0 =C2=A0while (!qemu_system_ready)
> =C2=A0 =C2=A0 =C2=A0 =C2=A0qemu_cond_wait(&qemu_system_cond);
> =C2=A0 =C2=A0pthread_mutex_unlock(&qemu_mutex);
>
> Shouldnt this cover the cpu hotplug case too? Perhaps have:
>
> =C2=A0 =C2=A0/* wait for machine initialization */
> =C2=A0 =C2=A0while (!qemu_system_ready)
> =C2=A0 =C2=A0 =C2=A0 =C2=A0qemu_cond_wait(&qemu_system_cond);
> =C2=A0 =C2=A0/* wait for vcpu initialization */
> =C2=A0 =C2=A0while (!env->initialized)
> =C2=A0 =C2=A0 =C2=A0 =C2=A0qemu_cond_wait(&qemu_system_cond);
> =C2=A0 =C2=A0pthread_mutex_unlock(&qemu_mutex);
>
> And then set env->initialized when the cpu is good to go.
>>From my understanding, all this is only useful when the whole machine
is starting, since they are global locks that wait for a system wide condit=
ion.

This is not the case with cpu hotplug, since the box is already on.

>
> Because there could be other dependencies other than APIC
> initialization, for eg in pc_new_cpu
>
> =C2=A0 =C2=A0 =C2=A0 =C2=A0if (cpu !=3D 0)
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0env->halted =3D 1;

it is okay for the cpu to be halted. Btw, I believe this should be
moved inside cpu init.