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Re: [Qemu-devel] [PATCH] dump-guest-memory.py: fix "You can't do that wi


From: Marc-André Lureau
Subject: Re: [Qemu-devel] [PATCH] dump-guest-memory.py: fix "You can't do that without a process to debug"
Date: Tue, 12 Dec 2017 12:17:35 -0500 (EST)


----- 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 <address@hidden>
> > ---
> >  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
> 



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