linux-kernel - Re: [Fastboot] [PATCH 1/1] Allow i386 crash kernels to handle x86

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Date:	Fri, 16 Mar 2007 11:40:07 +0900
From:	"Magnus Damm" <magnus.damm@...il.com>
To:	Horms <horms@...ge.net.au>
Cc:	"Vivek Goyal" <vgoyal@...ibm.com>, fastboot@...ts.osdl.org,
	linux-kernel@...r.kernel.org,
	"Ian Campbell" <Ian.Campbell@...source.com>
Subject: Re: [Fastboot] [PATCH 1/1] Allow i386 crash kernels to handle x86_64 dumps

On 3/16/07, Horms <horms@...ge.net.au> wrote:
> On Thu, Mar 15, 2007 at 06:56:16PM +0530, Vivek Goyal wrote:
> > On Thu, Mar 15, 2007 at 12:22:57PM +0000, Ian Campbell wrote:
> > > On Thu, 2007-03-15 at 11:17 +0530, Vivek Goyal wrote:
> > > > > > But I think changing this macro might run into issues. It is
> > > > > > being used at few places in kernel, for example while loading
> > > > > > module. This will essentially mean that we allow loading 64bit
> > > > > > x86_64 modules on 32bit i386 systems?
> > >
> > > Yes, not sure how I missed that fact...
> > >
> > > > Kexec will also not allow loading an x86_64 kernel on a 32bit machine.
> > >
> > > For crash kernel only or for regular kexec too?
> > >
> >
> > I think for both. One of the possible reasons I think is that one never
> > knows is underlying machine has got 64bit extensions or not. So even if
> > we load the kernel it will never boot. Secondly, we might not be able to
> > handle 64bit address in 32bit kernel/user space?
>
> Perhaps I am miss-understanding what you are saying, but I do
> recally kexecing from 32->64 and 64->32 bit kernels on x86_64 hardware.
> I can run these checks again if it helps.

I recall kexecing a bzImage for x86_64 on i386, but I'm not 100% sure.
I think it worked because the bzImage loader code was regular 32 bit
x86 code, but that may be wrong as well.

> Won't the above change break non i386 archtectures as
> vmcore_elf_check_arch_cross isn't defined for them?

Right. And maybe it's a good idea to make sure that this feature is
actually supported by kexec-tools before adding code to the kernel?

My gut feeling about this is that you are begging for trouble. The
kexec/kdump solution is fragile just by itself, and trying to go
between architectures is just going to be painful.

/ magnus
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