Re: [Crash-utility] [Qemu-devel] uniquely identifying KDUMP files that originate from QEMU

Thanks Petr and Laszlo for entertaining my questions. I've got one last one if you have the time. On 11/12/2014 09:10 AM, Laszlo Ersek wrote: > On 11/12/14 14:26, Petr Tesarik wrote: >> On Wed, 12 Nov 2014 08:18:04 -0500 >> Christopher Covington <cov(a)codeaurora.org&gt; wrote: >> >>> On 11/12/2014 03:05 AM, Petr Tesarik wrote: >>>> On Tue, 11 Nov 2014 12:27:44 -0500 >>>> Christopher Covington <cov(a)codeaurora.org&gt; wrote: >>>> >>>>> On 11/11/2014 06:22 AM, Laszlo Ersek wrote: >>>>>> (Note: I'm not subscribed to either qemu-devel or the kexec list; please >>>>>> keep me CC'd.) >>>>>> >>>>>> QEMU is able to dump the guest's memory in KDUMP format (kdump-zlib, >>>>>> kdump-lzo, kdump-snappy) with the "dump-guest-memory" QMP command. >>>>>> >>>>>> The resultant vmcore is usually analyzed with the "crash" utility. >>>>>> >>>>>> The original tool producing such files is kdump. Unlike the procedure >>>>>> performed by QEMU, kdump runs from *within* the guest (under a kexec'd >>>>>> kdump kernel), and has more information about the original guest kernel >>>>>> state (which is being dumped) than QEMU. To QEMU, the guest kernel state >>>>>> is opaque. >>>>>> >>>>>> For this reason, the kdump preparation logic in QEMU hardcodes a number >>>>>> of fields in the kdump header. The direct issue is the "phys_base" >>>>>> field. Refer to dump.c, functions create_header32(), create_header64(), >>>>>> and "include/sysemu/dump.h", macro PHYS_BASE (with the replacement text >>>>>> "0"). >>>>>> >>>>>> http://git.qemu.org/?p=qemu.git;a=blob;f=dump.c;h=9c7dad8f865af3b778589dd... >>>>>> >>>>>> http://git.qemu.org/?p=qemu.git;a=blob;f=include/sysemu/dump.h;h=7e4ec5c7... >>>>>> >>>>>> This works in most cases, because the guest Linux kernel indeed tends to >>>>>> be loaded at guest-phys address 0. However, when the guest Linux kernel >>>>>> is booted on top of OVMF (which has a somewhat unusual UEFI memory map), >>>>>> then the guest Linux kernel is loaded at 16MB, thereby getting out of >>>>>> sync with the phys_base=0 setting visible in the KDUMP header. >>>>>> >>>>>> This trips up the "crash" utility. >>>>>> >>>>>> Dave worked around the issue in "crash" for ELF format dumps -- "crash" >>>>>> can identify QEMU as the originator of the vmcore by finding the QEMU >>>>>> notes in the ELF vmcore. If those are present, then "crash" employs a >>>>>> heuristic, probing for a phys_base up to 32MB, in 1MB steps. >>>>> >>>>> What advantages does KDUMP have over ELF? >>>> >>>> It's smaller (data is compressed), and it contains a header with some >>>> useful information (e.g. the crashed kernel's version and release). > > Another advantage is that all zero-filled pages are represented in the > kdump file by one shared zero page. > > The difference in speed of dumping is stunning. Would you expect using SHT_NOBITS to give a similar speedup to the ELF dumper?