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

On 11/12/14 15:48, Christopher Covington wrote: > 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? Sorry, I don't know what SHT_NOBITS is.