From: Wen Congyang <wency@xxxxxxxxxxxxxx>
Subject: Re: question about phys_base
Date: Thu, 16 Feb 2012 09:16:28 +0800
> At 02/16/2012 12:17 AM, Dave Anderson Wrote:
>>
>>
>> ----- Original Message -----
>>> Hi, Dave
>>>
>>> I am implementing a new dump command in the qemu. The vmcore's
>>> format is elf(like kdump). And I try to provide phys_base in
>>> the PT_LOAD. But if the os uses the first vcpu do kdump, the
>>> value of phys_base is wrong.
>>>
>>> I find a function x86_64_virt_phys_base() in crash's code.
>>> Is it OK to call this function first? If the function
>>> successes, we do not calculate phys_base according to PT_LOAD.
>>
>> I'm presuming that the qemu-generated ELF file is essentially
>> a "clone" of a kdump ELF file, and therefore the initialization
>> sequence would be:
>>
>> main()
>> machdep_init(PRE_GDB)
>> x86_64_init(PRE_GDB)
>> x86_64_calc_phys_base()
>>
>> where it should fall into this part:
>>
>> if ((vd = get_kdump_vmcore_data())) {
>> for (i = 0; i < vd->num_pt_load_segments; i++) {
>> phdr = vd->load64 + i;
>> if ((phdr->p_vaddr >= __START_KERNEL_map) &&
>> !(IS_VMALLOC_ADDR(phdr->p_vaddr))) {
>>
>> machdep->machspec->phys_base = phdr->p_paddr -
>> (phdr->p_vaddr & ~(__START_KERNEL_map));
>>
>> if (CRASHDEBUG(1)) {
>> fprintf(fp, "p_vaddr: %lx p_paddr: %lx -> ",
>> phdr->p_vaddr, phdr->p_paddr);
>> fprintf(fp, "phys_base: %lx\n\n",
>> machdep->machspec->phys_base);
>> }
>> break;
>> }
>> }
>>
>> return;
>> }
>>
>> Question: will the qemu-generated ELF header contain a PT_LOAD segment that
>> describes the mapped __START_KERNEL_map region?
>>
>> If the __START_KERNEL_map PT_LOAD segment does *not* exist, then the code above
>> would fall through to the "return", and I suppose that you could call
>> x86_64_virt_phys_base() there instead.
>>
>> If there *is* a __START_KERNEL_map PT_LOAD segment, are you saying that
>> the calculation above would incorrectly calculate phys_base?
>
> Because it is hard to calculate phys_base in qemu side. I try to do it like
> the function get_kernel_base() in qemu.c. But if the os uses the vcpu to do
> kdump, the phys_base is for the second kernel, not the first kernel. Another
> problem is that it is for linux, and we donot which the guest is.
>
For the another problem, I don't know whether the way of checking the
type of running OS that is typically used, exists now, how about
letting users to specify the format through command-line? For example
--elf or --os=linux. Users who try to generate vmcore must know what
kind of OS is running, so I guess they can choose proper ones.
Of couse, if such way exists, it should be used.
>>
>> Ideally, there would be some other "differentiator" between qemu-generated
>> and kdump-generated ELF headers -- while still being a KDUMP clone in all
>> other respects. (Maybe an ELF NOTE?) And then preferably, that differentiator
>> could be used to separate the code, i.e., something like:
>
> The qemu-generated ELF headers may be the same as kdump-generated ELF headers.
>
> Thanks
> Wen Congyang
kdump ELF vmcore has further VMCOREINFO.
$ readelf -n /media/pub3/vmcores/vmcore-2.6.35.14-106.fc14.x86_64-10000-threads
Notes at offset 0x000001c8 with length 0x00000838:
Owner Data size Description
CORE 0x00000150 NT_PRSTATUS (prstatus structure)
CORE 0x00000150 NT_PRSTATUS (prstatus structure)
VMCOREINFO 0x00000557 Unknown note type: (0x00000000)
But diskdump/netdump ELF vmcore doesn't, so crash could possibly get
confused against this.
OTOH, I think qemu's CPU state information, CPUX86State structure, is
very useful debugging information. Because kvmdump format has the same
information, if this information is lost, this can be thouht of as a
kind of feature regression. So, how adding the information as new note
information? Then, this can help crash to distinguish the vmcore from
the original kdump's.
Thanks.
HATAYAMA, Daisuke
>>
>> if (qemu_generated_ELF_kdump() {
>> x86_64_virt_phys_base();
>> return;
>> }
>>
>> if ((vd = get_kdump_vmcore_data())) {
>> for (i = 0; i < vd->num_pt_load_segments; i++) {
>> phdr = vd->load64 + i;
>> if ((phdr->p_vaddr >= __START_KERNEL_map) &&
>> ...
>>
>> Would that be possible?
>>
>> Dave
>>
>>
>
>
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