Re: [Crash-utility] question about phys_base

From: Wen Congyang <wency(a)cn.fujitsu.com&gt; 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 >> >> > >