On Tue, 11 Nov 2014 12:22:52 +0100 Laszlo Ersek <lersek at redhat.com> 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. > > Alas, the QEMU notes are not present in the KDUMP-format vmcores that > QEMU produces (they cannot be), Why? Since KDUMP format version 4, the complete ELF notes can be stored in the file (see offset_note, size_note fields in the sub-header). Petr Tesarik
