From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jan Kiszka <jan.kiszka@web.de>
Subject: Re: [PATCH] kvm-qemu: Proper vm_stop on debug events
Date: Fri, 23 May 2008 04:10:34 +0200
Message-ID: <4836279A.6060308@web.de>
References: <48360B4B.10105@web.de> <48361154.4080805@codemonkey.ws>
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Cc: kvm-devel <kvm@vger.kernel.org>
To: Anthony Liguori <anthony@codemonkey.ws>
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Anthony Liguori wrote:
> Jan Kiszka wrote:
>> When a vcpu exits after hitting a debug exception, we have to invoke
>> vm_stop(EXCP_DEBUG). But this has to take place over the io-thread.
>>
>> This patch introduces kvm_debug_stop_requested to signal this event, a=
nd
>> it takes care that the interrupted vcpu itself goes immediately into
>> stop state.
>>
>> Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
>> ---
>>  qemu/qemu-kvm.c |    9 ++++++++-
>>  1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> Index: b/qemu/qemu-kvm.c
>> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>> --- a/qemu/qemu-kvm.c
>> +++ b/qemu/qemu-kvm.c
>> @@ -58,6 +58,8 @@ pthread_t io_thread;
>>  static int io_thread_fd =3D -1;
>>  static int io_thread_sigfd =3D -1;
>> =20
>> +static int kvm_debug_stop_requested;
>> +
>>  =20
>=20
> Why use this instead of just keying off of exception_index =3D=3D EXCP_=
DEBUG?
>=20
>>  static inline unsigned long kvm_get_thread_id(void)
>>  {
>>      return syscall(SYS_gettid);
>> @@ -517,6 +519,10 @@ int kvm_main_loop(void)
>>              qemu_system_powerdown();
>>          else if (qemu_reset_requested())
>>          qemu_kvm_system_reset();
>> +    else if (kvm_debug_stop_requested) {
>> +        kvm_debug_stop_requested =3D 0;
>> +        vm_stop(EXCP_DEBUG);
>> +    }
>>      }
>> =20
>>      pause_all_threads();
>> @@ -529,7 +535,8 @@ static int kvm_debug(void *opaque, int v
>>  {
>>      CPUState *env =3D cpu_single_env;
>> =20
>> -    env->exception_index =3D EXCP_DEBUG;
>> +    kvm_debug_stop_requested =3D 1;
>> +    vcpu_info[vcpu].stopped =3D 1;
>>  =20
>=20
> This isn't quite right.  In the very least, you need to set stopping =3D=
 0
> and signal on the qemu_pause_cond.  Thinking it through more though, a
> breakpoint should stop all VCPUs, right?

It does already, give it a try.

>=20
> vm_stop(EXCP_DEBUG) will actually do this.  It invokes the
> vm_state_notify callbacks and the io-thread registers one.  I think you=

> should probably just issue vm_stop(EXCP_DEBUG) from kvm_debug.

As explained, it can't be called over the vcpu contexts (there is even
an assert() fence against it). This policy has been recently agreed on
while fixing other deadlock issues of qemu-kvm.

Jan


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