On 11/12/14 14:28, Christopher Covington wrote: > On 11/12/2014 08:26 AM, Petr Tesarik wrote: >> On Wed, 12 Nov 2014 08:18:04 -0500 >> Christopher Covington <cov at codeaurora.org> wrote: >> >>> 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? >> >> Not the same thing. With KDUMP, each page is compressed separately, so >> if a utility like crash needs a page from the middle, it can find it >> and unpack it immediately. If we had an ELF.xz, then the whole file >> must be unpacked before it can be used. And unpacking a few terabytes >> takes ... a while. ;-) > > Understood on the ELF.xz approach, but why couldn't each page (or maybe a > configurable size) be a SHF_COMPRESSED section? Perhaps it could, technically -- it's just not how Qiao Nuohan implemented the feature. I didn't research the background for this. Thanks Laszlo
