On 11/12/2014 03:05 AM, Petr Tesarik wrote: > On Tue, 11 Nov 2014 12:27:44 -0500 > Christopher Covington <cov at codeaurora.org> 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=9c7dad8f865af3b778589dd0847e450ba9a75b9d;hb=HEAD >>> >>> http://git.qemu.org/?p=qemu.git;a=blob;f=include/sysemu/dump.h;h=7e4ec5c7d96fb39c943d970d1683aa2dc171c933;hb=HEAD >>> >>> 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). What if the ELF dumper used SHF_COMPRESSED or could dump an ELF.xz? How does QEMU figure out the kernel version information? Thanks, Chris -- Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project
