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

(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.