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SubjectRe: [patch 0/9] kdump: Patch series for s390 support
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Hello Vivek,

I attached a document where the s390 port is described in more detail.
Perhaps this helps you to understand what want and what we are doing. If
not - just delete it :-)

Michael

On Thu, 2011-07-07 at 15:33 -0400, Vivek Goyal wrote:
> On Wed, Jul 06, 2011 at 11:24:47AM +0200, Michael Holzheu wrote:
> > Hello Vivec,
> >
> > On Tue, 2011-07-05 at 16:26 -0400, Vivek Goyal wrote:
> > > On Mon, Jul 04, 2011 at 07:09:22PM +0200, Michael Holzheu wrote:
> >
> > [snip]
> >
> > > I don't understand what is stand-alone dump tools and
> >
> > S390 stand-alone dump tools are independent mini operating systems that
> > are installed on disks or tapes. When a dump should be created, these
> > stand-alone dump tools are booted. All that they do is to write the dump
> > (current memory plus the CPU registers) to the disk/tape device.
> >
> > The advantage compared to kdump is that since they are freshly loaded
> > into memory they can't be overwritten in memory.
>
> > Another advantage is
> > that since it is different code, it is much less likely that the dump
> > tool will run into the same problem than the previously crashed kernel.
>
> I think in practice this is not really a problem. If your kernel
> is not stable enough to even boot and copy a file, then most likely
> it has not even been deployed. The very fact that a kernel has been
> up and running verifies that it is a stable kernel for that machine
> and is capable of capturing the dump.
>
> > Also the boot process ensures that the hardware is in a initialized
> > state.
>
> Who makes sure that hardware is in initiliazed state? Kdump kernel,
> stand alone kernel or BIOS.
>
> > And last but not least, with the stand-alone dump tools you can
> > dump early kernel problems which is not possible using kdump, because
> > you can't dump before the kdump kernel has been loaded with kexec.
> >
>
> That is one limitation but again if your kernel can't even boot,
> it is not ready to ship and it is more of a development issue and
> there are other ways to debug problems. So I would not worry too
> much about it.
>
> On a side note, few months back there were folks who were trying
> to enhance bootloaders to be able to prepare basic environment so
> that a kdump kernel can boot even in the event of early first
> kernel boot.
>
> > That were more or less the arguments, why we did not support kdump in
> > the past.
> >
> > In order to increase dump reliability with kdump, we now implemented a
> > two stage approach. The stand-alone dump tools first check via meminfo,
> > if kdump is valid using checksums. If kdump is loaded and healthy it is
> > started. Otherwise the stand-alone dump tools create a full-blown
> > stand-alone dump.
>
> kexec-tools purgatory code also checks the checksum of loaded kernel
> and other information and next kernel boot starts only if nothing
> has been corrupted in first kernel. So this additional meminfo strucutres
> and need of checksums sounds unnecessary. I think what you do need is
> that somehow invoking second hook (s390 specific stand alone kernel)
> in case primary kernel is corrupted.
>
> >
> > With this approach we still keep our s390 dump reliability and gain the
> > great kdump features, e.g. distributor installer support, dump filtering
> > with makedumpfile, etc.
> >
> > > why the existing
> > > mechanism of preparing ELF headers to describe all the above info
> > > and just passing the address of header on kernel commnad line
> > > (crashkernel=) will not work for s390. Introducing an entirely new
> > > infrastructure for communicating the same information does not
> > > sound too exciting.
> >
> > We need the meminfo interface anyway for the two stage approach. The
> > stand-alone dump tools have to find and verify the kdump kernel in order
> > to start it.
>
> kexec-tools does this verification already. We verify the checksum of
> all the loaded information in reserved area. So why introduce this
> meminfo interface.
>
> > Therefore the interface is there and can be used. Also
> > creating the ELF header in the 2nd kernel is more flexible and easier
> > IMHO:
> > * You do not have to care about memory or CPU hotplug.
>
> Reloading the kernel upon memory or cpu hotplug should be trivial. This
> does not justify to move away from standard ELF interface and creation
> of a new one.
>
> > * You do not have to preallocate CPU crash notes etc.
>
> Its a small per cpu area. Looks like otherwise you will create meminfo
> areas otherwise.
>
> > * It works independently from the tool/mechanism that loads the kdump
> > kernel into memory. E.g. we have the idea to load the kdump kernel at
> > boot time into the crashkernel memory (not via the kexec_load system
> > call). That would solve the main kdump problems: The kdump kernel can't
> > be overwritten by I/O and also early kernel problems could then be
> > dumped using kdump.
>
> Can you give more details how exactly it works. I know very little about
> s390 dump mechanism.
>
> When do you load kdump kernel and who does it?
>
> Who gets the control first after crash?
>
> To me it looked like that you regularly load kdump kernel and if that
> is corrupted then somehow you boot standalone kernel. So corruption
> of kdump kernel should not be a issue for you.
>
> Do you load kdump kenrel from some tape/storage after system crash. Where
> does bootloader lies and how do you make sure it is not corrupted and
> associated device is in good condition.
>
> To me we should not create a arch specific way of passing information
> between kernels. Stand alone kernel should be able to parse the
> ELF headers which contains all the relevant info. They have already
> been checksum verified.
>
> Thanks
> Vivek

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