Re: [patch 0/9] kdump: Patch series for s390 support

[Michael Holzheu <holzheu@linux.vnet.ibm.com>]

Hello Vivek,

On Fri, 2011-07-15 at 10:38 -0400, Vivek Goyal wrote:
> > > In user space I think one can modify the kexec-tools infrastrucuture a
> > > bit so that one is able to define an entry point in case checksum of
> > > loaded segment failes. Once you are loding kdump kernel, you can define
> > > that entry point. (And this would be jump to IPL etc.).
> > 
> > You mean to jump back into the crashed kernel code in case the kdump
> > checksum failed?
> 
> No. I meant jump to entry point so that one can IPL the dump tools. I
> am not sure how do initiate the IPL after panic. Similar thing needs
> to be done here. If it is as simple as jumping to some location in
> low memory, then purgatory should be able to do that. I think we
> shall have to figure out the details here.

We have a machine instruction to IPL a dump tool from a device. The
parameters (e.g. device number, or WWPN/LUN for SCSI devices) are
currently configured via a s390 sysfs interface and an etc config file.
In theory we could read the sysfs files or the config file from the
kexec tool and patch the parameters into the purgatory code. The user
would then have to restart kexec each time when the configuration is
changed.

> Basically I am saying that purgatory detected that kdump kernel is
> corrupted. In x86_64 we spin in inifinite loop as we don't have a
> backup plan. But s390 has a backup plan of being able to IPL dump
> tools.
> 
> Or in first step we can keep it even simpler. We can spin in infinite
> loop

Looping is probably not a good option in a hypervisor environment like
we have it on s390. At least we should load a disabled wait PSW.

> and wait for either hypervisor watchdog to kick in for automatic
> IPL or wait for operator intervention.
> That would simplify it even
> further. 
> > 
> > In the meantime I was looking a bit more into the kexec code to find
> > out, what we would have to do, if we use the preallocated ELF header as
> > you want us to do. With our actual solution, we do not have to reserve
> > any special areas for the kdump kernel. Now we have to reserve the ELF
> > header. So what are the options?
> 
> ELF headers go into same memory area as kdump kenrel.

sure

> Anyway you are
> doing to reserve memory for kdump kernel and ELF headers will go 
> right there.
> Once you swap the kernel I think ELF headers continue to remain in
> original location. Or may be you can move ELF headers too depending
> on what turns out to be easier.
> 
> > 
> > The x86 implementation uses a kernel parameter "memmap=exactmap" to do
> > that.
> 
> It tells second kernel to use a memory map defined on command line.
> Kexec-tools prepares this memory map with the help of memap= options. This
> is to limit the memory second kernel use to boot into so that it does not
> overwrite in any piece of memory used by first kernel.

And to reserve the ELF header that is prepared by kexec tools, no?

> In your case I think you shall have to do little more so that second
> kernel also seems some of the lower memory areas so that later swapping
> of kernel can be done.

After the swap the ELF header is contained in the same memory than the
kdump kernel. When the kdump kernel starts, the ELF header has to be
saved from being overwritten (as kernel and ramdisk). I get the address
from the "elfcorehdr=" kernel parameter. How will I get the size?
Looking at the ia64 and x86 implementations I have the feeling there are
different mechanism available to do that.

> 
> > 
> > On ia64 - if I understood the code correctly - they seem to pass a kdump
> > segment "EFI_memmap" to the kdump kernel that contains information about
> > all loaded kexec segments. With this segment they can find out the size
> > of the ELF header segment in the kdump kernel and then do the memory
> > reservation at boot time. Is that correct?
> 
> Sorry, I don't know the details of IA64. May be somebody else on the list
> can pitch in with some clarifications here.

For me it looks like a mechanism where a block of information is
prepared by kexec tools and a pointer to that block is passed somehow to
the second kernel. I would assume that the definition of this block is
ia64 kernel ABI. 

See kernel:
* arch/ia64/kernel/setup.c: reserve_elfcorehdr()
* arch/ia64/kernel/head.S: ia64_boot_param

kexec tools:
* kexec/arch/ia64/kexec-elf-ia64.c: efi_memmap_buf
* purgatory/arch/ia64/entry.S: __boot_param_base

Michael