On 2012-02-15 10:47, HATAYAMA Daisuke wrote:
> From: Wen Congyang <we...@cn.fujitsu.com>
> Subject: Re: [RFC][PATCH 05/16 v6] Add API to get memory mapping
> Date: Wed, 15 Feb 2012 17:41:15 +0800
>
>> At 02/15/2012 05:17 PM, Jan Kiszka Wrote:
>>> On 2012-02-15 05:07, Wen Congyang wrote:
>>>> At 02/15/2012 01:21 AM, Jan Kiszka Wrote:
>>>>> On 2012-02-09 04:22, Wen Congyang wrote:
>>>>>> Add API to get all virtual address and physical address mapping.
>>>>>> If there is no virtual address for some physical address, the virtual
>>>>>> address is 0.
>>>>>>
>>>>>> Signed-off-by: Wen Congyang <we...@cn.fujitsu.com>
>>>>>> ---
>>>>>> memory_mapping.c | 65
>>>>>> ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>>>> memory_mapping.h | 1 +
>>>>>> 2 files changed, 66 insertions(+), 0 deletions(-)
>>>>>>
>>>>>> diff --git a/memory_mapping.c b/memory_mapping.c
>>>>>> index d83b7d7..fc0ddee 100644
>>>>>> --- a/memory_mapping.c
>>>>>> +++ b/memory_mapping.c
>>>>>> @@ -128,3 +128,68 @@ void free_memory_mapping_list(MemoryMappingList
>>>>>> *list)
>>>>>>
>>>>>> list->num = 0;
>>>>>> }
>>>>>> +
>>>>>> +void get_memory_mapping(MemoryMappingList *list)
>>>>>> +{
>>>>>> + CPUState *env;
>>>>>> + MemoryMapping *memory_mapping;
>>>>>> + RAMBlock *block;
>>>>>> + ram_addr_t offset, length;
>>>>>> +
>>>>>> + last_mapping = NULL;
>>>>>> +
>>>>>> + for (env = first_cpu; env != NULL; env = env->next_cpu) {
>>>>>> + cpu_get_memory_mapping(list, env);
>>>>>
>>>>> Hmm, is the CPU number recorded along with the mappings? I mean, how
>>>>> could crash tell them apart afterward if they are contradictory? This
>>>>> way, they are just thrown in the same bucket, correct?
>>>>>
>>>>> Even if crash or gdb aren't prepared for cpu/thread-specific mappings,
>>>>> could we already record that information for later use? Or would it
>>>>> break compatibility with current versions?
>>>>
>>>> crash does not need this information. It only needs the physical address
>>>> stored in PT_LOAD.
>>>
>>> So crash does not support viewing memory through the eyes of different
>>> CPUs? OK.
>>>
>>>>
>>>> gdb needs the virtual address and physical address stored in PT_LOAD.
>>>>
>>>> If the address is in the kernel space, the virtual address and physical
>>>> address mapping should be the same. I collect the mapping information
>>>> from all vcpus, because the OS may enter the second kernel. In this case,
>>>> IIRC(according to my test result, but I don't remeber clearly), gdb's bt
>>>> can output the backtrace in the first kernel if the OS does not use the
>>>> first vcpu to do kdump. otherwise gdb's bt can output the backtrace in
>>>> the second kernel.
>>>
>>> gdb could only make proper use of the additional mappings if they are
>>> not contradictory (which can easily happen with user space processes) or
>>> the cpu context is additionally provided so that views can be switched
>>> via the "thread N" command. So far, QEMU's gdbstub does this for gdb
>>> when it requests some memory over the remote connection. I bet gdb
>>> requires some extension to exploit such information offline from a core
>>> file, but I'm also sure that this will come as the importance of gdb for
>>> system level debugging will rise.
>>>
>>> Therefore my question: is there room to encode the mapping relation to a
>>> CPU/thread context?
>>
>> I donot know. But I think the answer is no, because there is no filed
>> in the struct Elf32_Phdr/Elf64_Phdr to store the CPU/thread id.
>>
>
> See NT_PRSTATUS note, from which gdb knows what CPUs is related to
> what thread.
>
> For vmcore generated by kdump, NT_PRSTATUS notes is contained in the
> order corresponding to online cpus.
>
> If crash reads the vmcore generated by this command just as by kdump
> and not considering this, crash might be understanding each CPU
> information wrongly because qemu dump generated all possible CPUs.
If that note makes most sense to encode the mapping context, let's use
it and fix crash to be prepared for it.
Jan
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