[Crash-utility] question about phys_base

[HATAYAMA Daisuke]

From: Wen Congyang <wency at cn.fujitsu.com>
Subject: Re: [Crash-utility] question about phys_base
Date: Tue, 28 Feb 2012 16:36:59 +0800

> At 02/28/2012 02:30 PM, HATAYAMA Daisuke Wrote:
>> From: Wen Congyang <wency at cn.fujitsu.com>
>> Subject: Re: [Crash-utility] question about phys_base
>> Date: Tue, 28 Feb 2012 13:10:38 +0800
>> 
>>> At 02/27/2012 10:10 PM, Dave Anderson Wrote:
>> 
>>>>> The guest is in the second kernel(vcpu > 1)
>>>>> ]# readelf /tmp/vm2.save2 -l| grep 0xffffffff8
>>>>>   LOAD           0x0000000001017be0 0xffffffff81000000 0x0000000001000000
>>>>>   LOAD           0x0000000001017be0 0xffffffff81000000 0x0000000001000000
>>>>>   LOAD           0x0000000001017be0 0xffffffff81000000 0x0000000001000000
>>>>>   LOAD           0x0000000004017be0 0xffffffff81000000 0x0000000004000000
>>>>
>>>> Again, it's not clear why there are multiple segments with the same
>>>> same virtual address, but I'm guessing that the one segment that starts 
>>>> at 0x0000000004000000 is associated with the second kernel, and the other
>>>> ones are for vcpus that ran in the first kernel?
>>>>  
>>>>> The guest is in the second kernel(vcpu = 1)
>>>>> [root at ghost ~]# readelf /tmp/vm2.save3 -l| grep 0xffffffff8
>>>>>   LOAD           0x0000000004001e4c 0xffffffff81000000 0x0000000004000000
>>>>>
>>>>> I donot find differentiate qemu-genetated ELF headers from dump-generated ELF
>>>>> headers.
>>>>
>>>> Kdump-generated vmcores cannot have multiple START_KERNEL_map segments.
>>>> But with dumps where (vpcu = 1), there could be confusion since it's not obvious
>>>> if START_KERNEL_map region belongs to the first or second kernel.  
>>>>
>>>> That being the case, I don't see how this can be supported cleanly by the crash'
>>>> utility unless there is a NOTE, or some other obvious identifier, that absolutely
>>>> confirms that the dumpfile was qemu-generated.
>>>
>>> The note information stored in qemu-generated core:
>>> Program Headers:
>>>   Type           Offset             VirtAddr           PhysAddr
>>>                  FileSiz            MemSiz              Flags  Align
>>>   NOTE           0x000000000000edd0 0x0000000000000000 0x0000000000000000
>>>                  0x0000000000000590 0x0000000000000590         0
>>>
>>> I think its format is the same as kdump's vmcore. Does kdump-generated core's
>>> virtaddr is always 0? If so, What about to set virt_addr to -1 in qemu-generated
>>> core?
>>>
>> 
>> In general, such characteristic should not be used. You should prepare
>> a solid interface. Even if using them, it should be limited to as
>> workaround to avoid some issue.
>> 
>> Why not use qemu's CPU state? Include it as note information with good
>> name, and we can use it to distinguish which. Like:
>> 
>> $ readelf -n vmcore
>> 
>> Notes at offset 0x000001c8 with length 0x00000838:
>>   Owner         Data size       Description
>>   CORE          0x00000150      NT_PRSTATUS (prstatus structure)
>>   CORE          0x00000150      NT_PRSTATUS (prstatus structure)
>>   QEMU          0x00000557      Unknown note type: (0x00000000)
>> 
>> Or QEMUCPUState is better?
> 
> Good idea. I will try it, and hope gdb can also work.
> 

Tools basically ignore unknown notes. Looking into gdb, it appears to
ignore unknown information.

static bfd_boolean
elfcore_grok_note (bfd *abfd, Elf_Internal_Note *note)
{
  const struct elf_backend_data *bed = get_elf_backend_data (abfd);

  switch (note->type)
    {
    default:
      return TRUE;
<cut>

You might need to add new command to output contents of new note if
it's necessary.

Thanks.
HATAYAMA, Daisuke