[Crash-utility] question about phys_base

[HATAYAMA Daisuke]

From: Dave Anderson <anderson at redhat.com>
Subject: Re: question about phys_base
Date: Thu, 16 Feb 2012 10:39:04 -0500 (EST)

> 
> 
> ----- Original Message -----
>> From: Wen Congyang <wency at cn.fujitsu.com>
>> 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
> 
> Right -- that would be a good idea.  In fact I thought I read that
> someone on the qemu-devel list had suggested that Wen do just that.
> 
> Dave  
> 

Thanks. I've just noticed. I've not read the thread yet. That's better.

Thanks.
HATAYAMA, Daisuke