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

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Date:	Fri, 16 Mar 2007 08:48:08 +0900
From:	Horms <horms@...ge.net.au>
To:	Vivek Goyal <vgoyal@...ibm.com>
Cc:	Ian Campbell <Ian.Campbell@...source.com>, hbabu@...ibm.com,
	fastboot@...ts.osdl.org, linux-kernel@...r.kernel.org,
	Magnus Damm <magnus.damm@...il.com>
Subject: Re: [PATCH 1/1] Allow i386 crash kernels to handle x86_64 dumps

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.

> > > So how about something like vmcore_elf_allowed_cross_arch()? Vmcore
> > > code can continue to check elf_check_arch() and if that fails it can
> > > invoke vmcore_elf_allowed_cross_arch() to find out what cross arch are
> > > allowed for vmcore. 
> > 
> > Something like this?
> > 
> > Ian.
> > 
> > ---  
> > 
> > Allow i386 crash kernels to handle x86_64 dumps.
> > 
> > The specific case I am encountering is kdump under Xen with a 64 bit
> > hypervisor and 32 bit kernel/userspace. The dump created is a 64 bit
> > due to the hypervisor but the dump kernel is 32 bit in for maximum
> > compatibility.
> > 
> > It's possibly less likely to be useful in a purely native scenario but
> > I see no reason to disallow it.
> > 
> > Signed-off-by: Ian Campbell <ian.campbell@...source.com>
> > 
> > diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
> > index d960507..523e109 100644
> > --- a/fs/proc/vmcore.c
> > +++ b/fs/proc/vmcore.c
> > @@ -514,7 +514,7 @@ static int __init parse_crash_elf64_headers(void)
> >  	/* Do some basic Verification. */
> >  	if (memcmp(ehdr.e_ident, ELFMAG, SELFMAG) != 0 ||
> >  		(ehdr.e_type != ET_CORE) ||
> > -		!elf_check_arch(&ehdr) ||
> > +		!vmcore_elf_check_arch(&ehdr) ||
> >  		ehdr.e_ident[EI_CLASS] != ELFCLASS64 ||
> >  		ehdr.e_ident[EI_VERSION] != EV_CURRENT ||
> >  		ehdr.e_version != EV_CURRENT ||
> > diff --git a/include/asm-i386/kexec.h b/include/asm-i386/kexec.h
> > index 4dfc9f5..c76737e 100644
> > --- a/include/asm-i386/kexec.h
> > +++ b/include/asm-i386/kexec.h
> > @@ -47,6 +47,9 @@
> >  /* The native architecture */
> >  #define KEXEC_ARCH KEXEC_ARCH_386
> > 
> > +/* We can also handle crash dumps from 64 bit kernel. */
> > +#define vmcore_elf_check_arch_cross(x) ((x)->e_machine == EM_X86_64)
> > +
> 
> Ideal place for this probably should have been arch dependent crash_dump.h
> file. But we don't have one and no point introducing one just for this 
> macro.
> 
> This change looks good to me.

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

-- 
Horms
  H: http://www.vergenet.net/~horms/
  W: http://www.valinux.co.jp/en/

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