Date:	Wed, 20 Jul 2011 10:00:10 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>,
	ebiederm@...ssion.com, hbabu@...ibm.com, mahesh@...ux.vnet.ibm.com,
	oomichi@....nes.nec.co.jp, horms@...ge.net.au,
	heiko.carstens@...ibm.com, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [patch 0/9] kdump: Patch series for s390 support

On Tue, 19 Jul 2011 11:04:23 -0400
Vivek Goyal <vgoyal@...hat.com> wrote:

> On Mon, Jul 18, 2011 at 08:03:08PM +0200, Michael Holzheu wrote:
> > Hello Vivek,
> > 
> > On Mon, 2011-07-18 at 11:25 -0400, Vivek Goyal wrote:
> > > On Mon, Jul 18, 2011 at 04:44:13PM +0200, Michael Holzheu wrote:
> > > > On Mon, 2011-07-18 at 10:19 -0400, Vivek Goyal wrote:
> > > > > - Make sure these headers are not overwritten by newly booted kernel.
> > > > 
> > > > And that was my question: What is the best way to do that. E.g. we could
> > > > pass a 2nd kernel parameter "elfcorehdr_size", implement s390 boot
> > > > parameter or implement the memmap kernel parameter.
> > > 
> > > You could do that but I think a more generic parameter will make more
> > > sense.
> > > 
> > > - Either something along the lines of memmap=
> > > - Or excludemem=x@y
> > > - Or modify memory map in s390 specific bootloading protocol block etc.
> > 
> > Ok, understood. Thanks for the information.
> > 
> > We still have discussions here, if we could somehow implement our
> > original idea of triggering kdump by the stand-alone dump tools. Sorry
> > for being so stubborn :-(
> 
> What's the advantage of that. Why are we so stuborn about first passing
> the control to dump tools after panic()?

I wonder when you will finally get it: we are not talking about the simple
case of a panic. Not all problems of the system will show up as a panic.
We occasionally have systems that just stop dead in their tracks. And this
is where an external dump trigger comes into play. For s390 that is the
stand-alone dumper.

> The case of purgatory corruption is no different then panic() code
> and associated hook code corruption. 

That is true. All the different pieces of code need to be verified with
a checksum.

> It is a corner case and even if it gets corrupted you have other
> mechanisms to IPL dump tools and capture dump.

It is definitely not a corner case. We use the stand-alone dumper as a
trigger to either start kdump if the code for kdump has not been corrupted
or as a fallback to do a full dump. The catch here is that we need a way to
distinguish the two cases. And that is where the checksums come into play.
See?

> Why do you want to mix two mechanisms. What's the advantage of making
> even dump tools complicated and make it aware of a kernel binary
> object purgatory?

We do that so we can use kdump in situations where the system just drops
dead and does not go over panic.

> To me the simple interface is that there is no coupling between dump
> tools and kdump. If there is no coupling, then there is no need to
> exchange any information and no need to make any assumption about
> hard coded location where purgatory entry point, size and checksums
> are stored.

Without the coupling we would have to do a full dump in case of an
unresponsive system. No fun if you have lots of main memory.

> > 
> > So here comes the modified suggestion:
> > 
> > As requested by you we can pre-allocate the ELF header and use purgatory
> > as done on other architectures.
> > 
> > To allow the stand-alone dump tools as kdump triggers, we then only
> > would have to provide an s390 specific way to tell the stand-alone dump
> > tools:
> > 1. Entry point address into purgatory
> > 2. Address, size and checksum for purgatory
> > 
> > We could store address, size and checksum of the purgatory to a fixed
> > offset in the kdump kernel image. This can be done in the kexec tools
> > code.
> 
> I think this will require kernel changes also? Otherwise how would you
> store variables in kernel address space.

I would think that this would best be implemented in some arch backend
function that is called on kexec_load.

> Secondly, if the goal is to just be able to checksum purgatory also, then
> it probably should be done in a generic mannner so that kernel could
> checksum purgatory before jumping to it.

You still seem to assume that the code that does the checksumming is
included in the main kernel and gets executed with crash_kexec. This is
incorrect in case of an external dump trigger. And before the stand-alone
dumper branches to the purgatory code it better makes sure that it does
not execute random numbers, otherwise we would get no dump at all.
 
> > Then the dump tools only would need the crashkernel memory offset
> > to find all information. Then dump tools will verify purgatory and
> > afterwards jump to the purgatory code. Then purgatory verifies all kexec
> > segments. For s390, if this check fails, we return to caller
> > (stand-alone tools). If the check is ok, then purgatory code on s390
> > saves all registers to the preallocated ELF notes and starts kdump.
> 
> So far I really don't think that there is any need of involving dump
> tools here. By making it a requirement we are just making the design
> complex with no gains.

We think otherwise. The dump trigger via the stand-alone dump tool is
a central requirement for us. And the design impact is minimal with the
latest suggestion from Michael.

> > 
> > I think, this is all s390 specific and IMHO will not affect other
> > architectures at all.
> > 
> > What you as kdump framework maintainer would have to accept with this
> > solution is that it is allowed now to start kdump directly via purgatory
> > without using code from the old kernel (e.g. crash_kexec). This has as
> > implication that all things that the old kernel has to initialize for
> > kdump has to be done before the system crashes. Currently this is only
> > the initialization of vmcoreinfo.
> 
> when would you save vmcoreinfo? I guess I shall have to look at the
> patches.

That should be patch #4 from the series.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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