[v1] dump-guest-memory.py: fix "You can't do that without a process to debug"

[Qemu-devel] [PATCH] dump-guest-memory.py: fix "You can't do that without a process to debug"

Posted by Marc-André Lureau 8 years, 1 month ago

If the script is run with a core (no running process), it produces an
error:

(gdb)  dump-guest-memory /tmp/vmcore X86_64
guest RAM blocks:
target_start     target_end       host_addr        message count
---------------- ---------------- ---------------- ------- -----
0000000000000000 00000000000a0000 00007f7935800000 added       1
00000000000a0000 00000000000b0000 00007f7934200000 added       2
00000000000c0000 00000000000ca000 00007f79358c0000 added       3
00000000000ca000 00000000000cd000 00007f79358ca000 joined      3
00000000000cd000 00000000000e8000 00007f79358cd000 joined      3
00000000000e8000 00000000000f0000 00007f79358e8000 joined      3
00000000000f0000 0000000000100000 00007f79358f0000 joined      3
0000000000100000 0000000080000000 00007f7935900000 joined      3
00000000fd000000 00000000fe000000 00007f7934200000 added       4
00000000fffc0000 0000000100000000 00007f7935600000 added       5
Python Exception <class 'gdb.error'> You can't do that without a process to debug.:
Error occurred in Python command: You can't do that without a process
to debug.

Replace the function call with a variable. I tried to use static
function variables, as suggested by Laszlo in earlier reviews, but the
compiler always took the chance to optimize them away.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
 scripts/dump-guest-memory.py | 3 +--
 hw/misc/vmcoreinfo.c         | 4 ++++
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/scripts/dump-guest-memory.py b/scripts/dump-guest-memory.py
index 1af26c1a45..9dec796762 100644
--- a/scripts/dump-guest-memory.py
+++ b/scripts/dump-guest-memory.py
@@ -546,8 +546,7 @@ shape and this command should mostly work."""
         return None
 
     def add_vmcoreinfo(self):
-        vmci = '(VMCoreInfoState *)' + \
-               'object_resolve_path_type("", "vmcoreinfo", 0)'
+        vmci = 'vmcoreinfo_state'
         if not gdb.parse_and_eval("%s" % vmci) \
            or not gdb.parse_and_eval("(%s)->has_vmcoreinfo" % vmci):
             return
diff --git a/hw/misc/vmcoreinfo.c b/hw/misc/vmcoreinfo.c
index 31db57ab44..d429a4f7d6 100644
--- a/hw/misc/vmcoreinfo.c
+++ b/hw/misc/vmcoreinfo.c
@@ -31,6 +31,9 @@ static void vmcoreinfo_reset(void *dev)
     s->vmcoreinfo.host_format = cpu_to_le16(VMCOREINFO_FORMAT_ELF);
 }
 
+/* this variable is exported for gdb script dump-guest-memory.py */
+VMCoreInfoState *vmcoreinfo_state;
+
 static void vmcoreinfo_realize(DeviceState *dev, Error **errp)
 {
     VMCoreInfoState *s = VMCOREINFO(dev);
@@ -56,6 +59,7 @@ static void vmcoreinfo_realize(DeviceState *dev, Error **errp)
                              &s->vmcoreinfo, sizeof(s->vmcoreinfo), false);
 
     qemu_register_reset(vmcoreinfo_reset, dev);
+    vmcoreinfo_state = s;
 }
 
 static const VMStateDescription vmstate_vmcoreinfo = {
-- 
2.15.1.355.g36791d7216

Re: [Qemu-devel] [PATCH] dump-guest-memory.py: fix "You can't do that without a process to debug"

Posted by Laszlo Ersek 8 years, 1 month ago

On 12/12/17 17:35, Marc-André Lureau wrote:
> If the script is run with a core (no running process), it produces an
> error:
> 
> (gdb)  dump-guest-memory /tmp/vmcore X86_64
> guest RAM blocks:
> target_start     target_end       host_addr        message count
> ---------------- ---------------- ---------------- ------- -----
> 0000000000000000 00000000000a0000 00007f7935800000 added       1
> 00000000000a0000 00000000000b0000 00007f7934200000 added       2
> 00000000000c0000 00000000000ca000 00007f79358c0000 added       3
> 00000000000ca000 00000000000cd000 00007f79358ca000 joined      3
> 00000000000cd000 00000000000e8000 00007f79358cd000 joined      3
> 00000000000e8000 00000000000f0000 00007f79358e8000 joined      3
> 00000000000f0000 0000000000100000 00007f79358f0000 joined      3
> 0000000000100000 0000000080000000 00007f7935900000 joined      3
> 00000000fd000000 00000000fe000000 00007f7934200000 added       4
> 00000000fffc0000 0000000100000000 00007f7935600000 added       5
> Python Exception <class 'gdb.error'> You can't do that without a process to debug.:
> Error occurred in Python command: You can't do that without a process
> to debug.
> 
> Replace the function call with a variable.

Can you state, "replace the object_resolve_path_type() function call
with an extern variable"?

> I tried to use static
> function variables, as suggested by Laszlo in earlier reviews, but the
> compiler always took the chance to optimize them away.

Did you qualify them "volatile" too?

E.g., in the vmcoreinfo_realize() function:

  static VMCoreInfoState * volatile vmcoreinfo_state;
  ...
  vmcoreinfo_state = s;

(Note: it's the pointer itself that has to be volatile, not the
pointed-to object.)

If you did try this, but GCC optimized it away, then that's a GCC bug.
In that case I guess I can't object to the present approach any longer.
Can you please confirm?

> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  scripts/dump-guest-memory.py | 3 +--
>  hw/misc/vmcoreinfo.c         | 4 ++++
>  2 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/scripts/dump-guest-memory.py b/scripts/dump-guest-memory.py
> index 1af26c1a45..9dec796762 100644
> --- a/scripts/dump-guest-memory.py
> +++ b/scripts/dump-guest-memory.py
> @@ -546,8 +546,7 @@ shape and this command should mostly work."""
>          return None
>  
>      def add_vmcoreinfo(self):
> -        vmci = '(VMCoreInfoState *)' + \
> -               'object_resolve_path_type("", "vmcoreinfo", 0)'
> +        vmci = 'vmcoreinfo_state'
>          if not gdb.parse_and_eval("%s" % vmci) \
>             or not gdb.parse_and_eval("(%s)->has_vmcoreinfo" % vmci):
>              return
> diff --git a/hw/misc/vmcoreinfo.c b/hw/misc/vmcoreinfo.c
> index 31db57ab44..d429a4f7d6 100644
> --- a/hw/misc/vmcoreinfo.c
> +++ b/hw/misc/vmcoreinfo.c
> @@ -31,6 +31,9 @@ static void vmcoreinfo_reset(void *dev)
>      s->vmcoreinfo.host_format = cpu_to_le16(VMCOREINFO_FORMAT_ELF);
>  }
>  
> +/* this variable is exported for gdb script dump-guest-memory.py */
> +VMCoreInfoState *vmcoreinfo_state;
> +
>  static void vmcoreinfo_realize(DeviceState *dev, Error **errp)
>  {
>      VMCoreInfoState *s = VMCOREINFO(dev);
> @@ -56,6 +59,7 @@ static void vmcoreinfo_realize(DeviceState *dev, Error **errp)
>                               &s->vmcoreinfo, sizeof(s->vmcoreinfo), false);
>  
>      qemu_register_reset(vmcoreinfo_reset, dev);
> +    vmcoreinfo_state = s;
>  }
>  
>  static const VMStateDescription vmstate_vmcoreinfo = {
> 

Thanks!
Laszlo

Re: [Qemu-devel] [PATCH] dump-guest-memory.py: fix "You can't do that without a process to debug"

Posted by Marc-André Lureau 8 years, 1 month ago


----- Original Message -----
> On 12/12/17 17:35, Marc-André Lureau wrote:Hi
> > If the script is run with a core (no running process), it produces an
> > error:
> > 
> > (gdb)  dump-guest-memory /tmp/vmcore X86_64
> > guest RAM blocks:
> > target_start     target_end       host_addr        message count
> > ---------------- ---------------- ---------------- ------- -----
> > 0000000000000000 00000000000a0000 00007f7935800000 added       1
> > 00000000000a0000 00000000000b0000 00007f7934200000 added       2
> > 00000000000c0000 00000000000ca000 00007f79358c0000 added       3
> > 00000000000ca000 00000000000cd000 00007f79358ca000 joined      3
> > 00000000000cd000 00000000000e8000 00007f79358cd000 joined      3
> > 00000000000e8000 00000000000f0000 00007f79358e8000 joined      3
> > 00000000000f0000 0000000000100000 00007f79358f0000 joined      3
> > 0000000000100000 0000000080000000 00007f7935900000 joined      3
> > 00000000fd000000 00000000fe000000 00007f7934200000 added       4
> > 00000000fffc0000 0000000100000000 00007f7935600000 added       5
> > Python Exception <class 'gdb.error'> You can't do that without a process to
> > debug.:
> > Error occurred in Python command: You can't do that without a process
> > to debug.
> > 
> > Replace the function call with a variable.
> 
> Can you state, "replace the object_resolve_path_type() function call
> with an extern variable"?

ok

> 
> > I tried to use static
> > function variables, as suggested by Laszlo in earlier reviews, but the
> > compiler always took the chance to optimize them away.
> 
> Did you qualify them "volatile" too?
> 
> E.g., in the vmcoreinfo_realize() function:
> 
>   static VMCoreInfoState * volatile vmcoreinfo_state;
>   ...
>   vmcoreinfo_state = s;
> 
> (Note: it's the pointer itself that has to be volatile, not the
> pointed-to object.)
> 
> If you did try this, but GCC optimized it away, then that's a GCC bug.
> In that case I guess I can't object to the present approach any longer.
> Can you please confirm?
> 

It seems to work, but you get a gcc warning:

hw/misc/vmcoreinfo.c:38:39: warning: variable ‘vmcoreinfo_state’ set but not used [-Wunused-but-set-variable]

I'll declare it like that and send v2:
static VMCoreInfoState * volatile vmcoreinfo_state G_GNUC_UNUSED;

> > 
> > Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> > ---
> >  scripts/dump-guest-memory.py | 3 +--
> >  hw/misc/vmcoreinfo.c         | 4 ++++
> >  2 files changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/scripts/dump-guest-memory.py b/scripts/dump-guest-memory.py
> > index 1af26c1a45..9dec796762 100644
> > --- a/scripts/dump-guest-memory.py
> > +++ b/scripts/dump-guest-memory.py
> > @@ -546,8 +546,7 @@ shape and this command should mostly work."""
> >          return None
> >  
> >      def add_vmcoreinfo(self):
> > -        vmci = '(VMCoreInfoState *)' + \
> > -               'object_resolve_path_type("", "vmcoreinfo", 0)'
> > +        vmci = 'vmcoreinfo_state'
> >          if not gdb.parse_and_eval("%s" % vmci) \
> >             or not gdb.parse_and_eval("(%s)->has_vmcoreinfo" % vmci):
> >              return
> > diff --git a/hw/misc/vmcoreinfo.c b/hw/misc/vmcoreinfo.c
> > index 31db57ab44..d429a4f7d6 100644
> > --- a/hw/misc/vmcoreinfo.c
> > +++ b/hw/misc/vmcoreinfo.c
> > @@ -31,6 +31,9 @@ static void vmcoreinfo_reset(void *dev)
> >      s->vmcoreinfo.host_format = cpu_to_le16(VMCOREINFO_FORMAT_ELF);
> >  }
> >  
> > +/* this variable is exported for gdb script dump-guest-memory.py */
> > +VMCoreInfoState *vmcoreinfo_state;
> > +
> >  static void vmcoreinfo_realize(DeviceState *dev, Error **errp)
> >  {
> >      VMCoreInfoState *s = VMCOREINFO(dev);
> > @@ -56,6 +59,7 @@ static void vmcoreinfo_realize(DeviceState *dev, Error
> > **errp)
> >                               &s->vmcoreinfo, sizeof(s->vmcoreinfo),
> >                               false);
> >  
> >      qemu_register_reset(vmcoreinfo_reset, dev);
> > +    vmcoreinfo_state = s;
> >  }
> >  
> >  static const VMStateDescription vmstate_vmcoreinfo = {
> > 
> 
> Thanks!
> Laszlo
>

Re: [Qemu-devel] [PATCH] dump-guest-memory.py: fix "You can't do that without a process to debug"

Posted by Laszlo Ersek 8 years, 1 month ago

On 12/12/17 18:17, Marc-André Lureau wrote:
> 
> 
> ----- Original Message -----
>> On 12/12/17 17:35, Marc-André Lureau wrote:Hi
>>> If the script is run with a core (no running process), it produces an
>>> error:
>>>
>>> (gdb)  dump-guest-memory /tmp/vmcore X86_64
>>> guest RAM blocks:
>>> target_start     target_end       host_addr        message count
>>> ---------------- ---------------- ---------------- ------- -----
>>> 0000000000000000 00000000000a0000 00007f7935800000 added       1
>>> 00000000000a0000 00000000000b0000 00007f7934200000 added       2
>>> 00000000000c0000 00000000000ca000 00007f79358c0000 added       3
>>> 00000000000ca000 00000000000cd000 00007f79358ca000 joined      3
>>> 00000000000cd000 00000000000e8000 00007f79358cd000 joined      3
>>> 00000000000e8000 00000000000f0000 00007f79358e8000 joined      3
>>> 00000000000f0000 0000000000100000 00007f79358f0000 joined      3
>>> 0000000000100000 0000000080000000 00007f7935900000 joined      3
>>> 00000000fd000000 00000000fe000000 00007f7934200000 added       4
>>> 00000000fffc0000 0000000100000000 00007f7935600000 added       5
>>> Python Exception <class 'gdb.error'> You can't do that without a process to
>>> debug.:
>>> Error occurred in Python command: You can't do that without a process
>>> to debug.
>>>
>>> Replace the function call with a variable.
>>
>> Can you state, "replace the object_resolve_path_type() function call
>> with an extern variable"?
> 
> ok
> 
>>
>>> I tried to use static
>>> function variables, as suggested by Laszlo in earlier reviews, but the
>>> compiler always took the chance to optimize them away.
>>
>> Did you qualify them "volatile" too?
>>
>> E.g., in the vmcoreinfo_realize() function:
>>
>>   static VMCoreInfoState * volatile vmcoreinfo_state;
>>   ...
>>   vmcoreinfo_state = s;
>>
>> (Note: it's the pointer itself that has to be volatile, not the
>> pointed-to object.)
>>
>> If you did try this, but GCC optimized it away, then that's a GCC bug.
>> In that case I guess I can't object to the present approach any longer.
>> Can you please confirm?
>>
> 
> It seems to work, but you get a gcc warning:
> 
> hw/misc/vmcoreinfo.c:38:39: warning: variable ‘vmcoreinfo_state’ set but not used [-Wunused-but-set-variable]
> 
> I'll declare it like that and send v2:
> static VMCoreInfoState * volatile vmcoreinfo_state G_GNUC_UNUSED;

Awesome; thank you very much!

(If "G_GNUC_UNUSED" is otherwise accepted practice in the QEMU tree;
that is -- it does seem to be widely used.)

... please don't forget to update the commit message again (the variable
will have block scope, static storage duration, and *no* linkage -- so
it shouldn't be called "extern" like I originally suggested.)

Cheers!
Laszlo

> 
>>>
>>> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
>>> ---
>>>  scripts/dump-guest-memory.py | 3 +--
>>>  hw/misc/vmcoreinfo.c         | 4 ++++
>>>  2 files changed, 5 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/scripts/dump-guest-memory.py b/scripts/dump-guest-memory.py
>>> index 1af26c1a45..9dec796762 100644
>>> --- a/scripts/dump-guest-memory.py
>>> +++ b/scripts/dump-guest-memory.py
>>> @@ -546,8 +546,7 @@ shape and this command should mostly work."""
>>>          return None
>>>  
>>>      def add_vmcoreinfo(self):
>>> -        vmci = '(VMCoreInfoState *)' + \
>>> -               'object_resolve_path_type("", "vmcoreinfo", 0)'
>>> +        vmci = 'vmcoreinfo_state'
>>>          if not gdb.parse_and_eval("%s" % vmci) \
>>>             or not gdb.parse_and_eval("(%s)->has_vmcoreinfo" % vmci):
>>>              return
>>> diff --git a/hw/misc/vmcoreinfo.c b/hw/misc/vmcoreinfo.c
>>> index 31db57ab44..d429a4f7d6 100644
>>> --- a/hw/misc/vmcoreinfo.c
>>> +++ b/hw/misc/vmcoreinfo.c
>>> @@ -31,6 +31,9 @@ static void vmcoreinfo_reset(void *dev)
>>>      s->vmcoreinfo.host_format = cpu_to_le16(VMCOREINFO_FORMAT_ELF);
>>>  }
>>>  
>>> +/* this variable is exported for gdb script dump-guest-memory.py */
>>> +VMCoreInfoState *vmcoreinfo_state;
>>> +
>>>  static void vmcoreinfo_realize(DeviceState *dev, Error **errp)
>>>  {
>>>      VMCoreInfoState *s = VMCOREINFO(dev);
>>> @@ -56,6 +59,7 @@ static void vmcoreinfo_realize(DeviceState *dev, Error
>>> **errp)
>>>                               &s->vmcoreinfo, sizeof(s->vmcoreinfo),
>>>                               false);
>>>  
>>>      qemu_register_reset(vmcoreinfo_reset, dev);
>>> +    vmcoreinfo_state = s;
>>>  }
>>>  
>>>  static const VMStateDescription vmstate_vmcoreinfo = {
>>>
>>
>> Thanks!
>> Laszlo
>>