[Crash-utility] kdump format may be updated
Dave Anderson
anderson at redhat.com
Wed Oct 25 14:02:38 UTC 2006
Magnus Damm wrote:
> On Tue, 2006-10-24 at 09:45 -0400, Dave Anderson wrote:
> > Magnus Damm wrote:
> > > The idea is that the crash_notes contents in the Xen hypervisor
> > space
> > > contains registers indexed by physical cpu number.
> > >
> > > It is possible to locate the crashing physical cpu by looking up a
> > > global variable in hypervisor symbol, and from there it should be
> > > possible to backtrack and find the domain pseudo-phys to virt
> > mapping
> > > table. I say "should" because it is probably pretty hairy.
> > >
> >
> > Actually, given that the crash utility is only interested in the
> > specifics of the dom0 kernel, it has no interest in physical
> > cpus. If you're specifically interested in debugging a crash
> > that occurred while operating in the xen binary, you're going
> > to want to use gdb on the vmcore file with xen-syms-xxx
> > namelist file. You can still run crash on the same vmcore to
> > find out what was going on in the dom0 kernel, but there's
> > no awareness of the xen hypervisor underpinnings; you'll
> > just get the state of the dom0 kernel at the time of the crash.
>
> Exactly. But for gdb to work we need to provide crash notes to the xen
> vmcore file - and with physical cpu crash notes in this case. I just
> hacked up some code to do this and it seems like the default number of
> cpus in xen-unstable is set to 32. That's 32 crash notes.
>
That's fine -- the crash utility will ignore the registers in the
NT_PRSTATUS notes if it can determine that the vmcore is
a hypervisor/dom0/kdump-type dumpfile. In my prototype, it
does just that when it sees the unique NT_XEN_DOM0_CR3 note.
(crash doesn't really need the NR_PRSTATUS register contents,
because it can find them elsewhere if need be.)
>
> > But I would guess-timate that the majority of the crashes are
> > going to have occurred in the dom0 kernel, and not while
> > running in the hypervisor.
>
> Yeah, given the number of lines of code...
>
> > Now, given that that the crash_notes context contains registers
> > that are indexed by the physical cpu number, well, that's not
> > helpful to crash's needs with respect to dom0. That's why you
> > guys must have created the additional NT_XEN_DOM0_CR3
> > ELF note.
>
> I'm note sure exactly why we created it - I thought it was because you
> wanted it - but I'm pretty sure Moriwaka-sans tool can locate things
> without it.
>
That's exactly right -- I requested dom0's cr3 or pfn_to_mfn_list_list
value, and Moriwaka-san provided me with a prototype vmcore that
contained the NT_XEN_DOM0_CR3 ELF note.
I presume that the i386 vmcore he sent me was also usable with
gdb and xen-syms. Here's what the header looks like:
$ readelf -a vmcore
ELF Header:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: CORE (Core file)
Machine: Intel 80386
Version: 0x1
Entry point address: 0x0
Start of program headers: 52 (bytes into file)
Start of section headers: 0 (bytes into file)
Flags: 0x0
Size of this header: 52 (bytes)
Size of program headers: 32 (bytes)
Number of program headers: 5
Size of section headers: 0 (bytes)
Number of section headers: 0
Section header string table index: 0
There are no sections in this file.
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
NOTE 0x0000d4 0x00000000 0x00000000 0x00190 0x00190 0
LOAD 0x000264 0xc0000000 0x00000000 0xa0000 0xa0000 RWE 0
LOAD 0x0a0264 0xc0100000 0x00100000 0x3f00000 0x3f00000 RWE 0
LOAD 0x3fa0264 0xc8000000 0x08000000 0x30000000 0x30000000 RWE 0
LOAD 0x33fa0264 0xffffffff 0x38000000 0x7ffb000 0x7ffb000 RWE 0
There is no dynamic segment in this file.
There are no relocations in this file.
There are no unwind sections in this file.
No version information found in this file.
Notes at offset 0x000000d4 with length 0x00000190:
Owner Data size Description
CORE 0x00000090 NT_PRSTATUS (prstatus structure)
Xen Domanin-0 CR3 0x00000004 Unknown note type: (0x10000001)
CORE 0x00000090 NT_PRSTATUS (prstatus structure)
Xen Domanin-0 CR3 0x00000004 Unknown note type: (0x10000001)
$
>
> > I guess I understand why you feel it's a burden to continue
> > the maintenance of such a thing, but given that the panic can
> > occur either while operating in the dom0 kernel or while in
> > the xen hypervisor code, it makes perfect sense (to me) to
> > make a minimal effort by including an indication of the dom0
> > cr3 or dom0 pfn_to_mfn_frame_list_list value in the vmcore.
> > Perhaps you consider it a case of the tail wagging the dog,
> > but to me, it would be more a case of accomodating the needs
> > of the consumer... ;-)
> >
>
> We should definately have something, we just need to figure out what. =)
>
> > >
> > > Our internal interfaces are not particularly clean at the moment. We
> > > have code that keeps the crash_notes in the hypervisor, but passes
> > the
> > > physical addresses (or machine addresses in xen lingo) for the notes
> > all
> > > the way down to kexec-tools in dom0 user space. These addresses are
> > then
> > > used to create the ELF headers. dom0 only knows about VCPU:s, but
> > > because we are creating a system-wide crash dump we want to use
> > physical
> > > cpus. So down in user space we then need to create a mapping between
> > > physical cpu:s and VCPU:s. And can we be sure that dom0 has all cpus
> > > available as VCPU:s?
> > >
> >
> > I don't care about that. All I need is a starting point for
> > translating dom0
> > kernel virtual addresses. And that is either a dom0 cr3 value or the
> > domain's pfn_to_mfn_frame_list_list value.
>
> I understand that. I'm sure we can figure out a good solution.
>
> > >
> > > > But again, there's no easy way for the crash utility to dig
> > > > them out of a completely foreign binary's.
> > >
> > > No, but that's because your tool is missing knowledge about the
> > binary
> > > right? =) Is there any easy way out... No! =) Or maybe there is?
> >
> > No!
> >
> > >
> > >
> > > I hope we can find a good balance between your code and ours. Maybe
> > a
> > > relatively fair balance could be that we provide per-physical cpu
> > > pointers to some virtual to physical mapping tables which should be
> > easy
> > > to parse for your tool, but in return your tool doesn't depend on
> > > finding register information using the note program headers in the
> > ELF
> > > header...
> >
> > Now we're getting complex -- I'm pretty sure I don't know what
> > you're talking about here... Or how it can possibly lead to a dom0
> > cr3 or pfn_to_mfn_frame_list_list value?
>
> Let's put it like this: Your tool, does it use xen-syms today? How
> difficult would it be to use lookup a symbol in hypervisor space? And
> how do you feel about requiring xen-syms to be able to parse dom0?
>
Definitely not. In the interest of simplicity, the idea is to keep
things the way they have always been, i.e., "crash vmlinux vmcore",
and having to drag out the relevant xen-syms binary defeats that
concept. And that's not to mention having to read it, figure out
how to translate hypervisor virtual addresses, then navigate around
the vmcore to find the location of a symbol, having to deal with the
following of data structures that may change over time -- to eventually
find what's needed...
>
> I'm not talking about your tool walking arch-dependent cross linked data
> structures in the xen hypervisor, just basic symbol lookup.
>
But I'm not sure what good would that do? The "domain_list" symbol
is only the beginning of the structure chain...
>
> > > That's good, isn't it? If I've understood things right it's possible
> > to
> > > locate the data you need using the domain list symbol?
> >
> > Yeah...
> >
> > To clarify, it's possible for *you*, i.e., the kexec/kdump code, to
> > locate
> > the data that way. The crash utility, using the vmlinux/vmcore file
> > pair, doesn't know anything about what the "domain_list" is, the
> > structures that it uses/links-to. And even if it did, it wouldn't
> > know
> > how to find it in the vmcore file.
>
> Hm...
>
> > > Yeah, I agree that navigating around those structures seems rather
> > > painful. But OTOH, if you want to know things that only the
> > internals
> > > can tell you, you need to be able to parse them, right? But maybe
> > you
> > > only want to cover the "simple" dom0 case. (Simple yeah right)
> > >
> >
> > That's right -- crash is *only* interested in the dom0 case; again
> > it's clueless about the hypervisor, and rightly so.
> >
> > It's just such a unique case. It's like trying to debug "ls" using
> > a "cat" binary, where the core file is usable for debugging either
> > one.
>
> I will continue working on cleaning up the code. Some of the changes
> that have been made or are going to happen are:
>
> - Separate dom0 notes from hypervisor notes.
Looks like a reasonable spot to stuff cr3 (or pfn_to_mfn_frame_list_list)?
(i.e. stuffing the dom0 cr3 at the end of the dom0 NT_PRSTATUS
register dump(s), like your current patch talks about)
>
> - Xen vmcore files should have per physical cpu hypervisor crash_notes.
> - These hypervisor notes should be pointed out by the program header.
> - Xen vmcore files should have the hypervisor in a PT_LOAD segment.
> - Xen vmcore files should have PT_LOAD headers for RAM, but with VA = 0.
With respect to the PT_LOAD segments, the crash utility pretty much
ignores the p_vaddr values -- it's particularly interested in the
p_paddr values. And once able to translate a unity-mapped kernel
virtual address into a physical address, it can find that physical
address by checking the p_paddr/p_filesz values in each PT_LOAD
segment. (Again -- that only works with xen kernels if the pseudo
physical address can be translated into a machine address, and then
a vmcore file location -- hence the pre-requisite requirement for
either the dom0 cr3 or pfn_to_mfn_list_list value.)
The one exception to "ignore-the-p_vaddr" rule is for the new
relocatable x86_64 kernels, where the PT_LOAD mapping for the
kernel's __START_KERNEL_map region needs to be looked at in
order to figure out where the kernel was physically loaded,
because in relocatable kernels, unity-mapping can no longer be
done by simply stripping off the PAGE_OFFSET value. Relocatable
kernels are being introduced so that there is no need for separate
"first-kernel" and the secondary kexec'd "kdump-kernel".
But AFAIK, the new kernel relocation scheme shouldn't infect xen
x86_64 kernels, so I don't believe that would ever be an issue.
In other words, xen kernels should remain unity-mapped in the
traditional manner, albeit with pseudo-physical addresses instead
of machine addresses. Again, I *believe* that to be true, since
there's no reason to relocate them.
>
> - Xen vmcore files should provide crash with something like CR3.
Or this would be good... ;-)
>
> - kexec-tools will be made xen aware.
>
> I'm sorry if we've been going over and over about this, but I'm a bit
> confused by the current status, what we want to have and if that is
> going to work. The points above show the direction. Please shout if you
> think they sound bad.
Nothing sounds bad to me! What would be really helpful is if,
during your development, you could provide me with sample
vmlinux/vmcore pairs that I could work with? Just so I don't
have to scramble at the end of it all, and only then find out
that there's a "gotcha" that we're not considering.
Thanks,
Dave
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