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Date: Wed, 17 Feb 2010 08:49:46 -0500 (EST)
From: Dave Anderson <anderson@redhat.com>
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Subject: Re: FYI: x86_64 bug when using gdb with vmcore
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To: Simon Horman <horms@verge.net.au>
Cc: kexec@lists.infradead.org


----- "Simon Horman" <horms@verge.net.au> wrote:

> On Fri, Feb 05, 2010 at 11:53:18AM -0500, Dave Anderson wrote:
> > 
> > The kexec/arch/x86_64/crashdump-x86_64.h file contains a
> > stale PAGE_OFFSET value.  In 2.6.27 it was changed from
> > 0xffff810000000000UL to 0xffff880000000000UL.  This is
> > only a problem when using gdb with the vmlinux/vmcore
> > pair, because gdb relies upon the PT_LOAD segment's p_vaddr
> > values in the ELF header to be correct.
> > 
> > Anyway, in the RHEL6 kexec-tools, this simple patch 
> > was made:
> > 
> > -#define PAGE_OFFSET            0xffff810000000000UL
> > +#define PAGE_OFFSET            0xffff880000000000UL
> > 
> > which is OK since the RHEL6 version of kexec-tools
> > will only be used with RHEL6 kernels.  
> > 
> > But for backwards compatibility, the better way to do 
> > it would be to check the utsname of the running kernel, 
> > and use the right value.
> > 
> > But again, this is only required for gdb usage.  The 
> > crash utility (and makedumpfile) make kernel version checks 
> > to determine which x86_64 PAGE_OFFSET value is applicable,
> > and don't rely on the ELF header p_vaddr values for
> > the unity-mapped regions.  
> 
> Hi Dave,
> 
> Something like this?

Looks good to me -- all except for the second "if" statement/typo below:  

> +int arch_init(void)
> +{
> +	int kv;
> +
> +	kv = kernel_version();
> +	if (kv < 0)
> +		return -1;
> +
> +	if (kv < KERNEL_VERSION(2, 4, 27))
> +		page_offset = PAGE_OFFSET_PRE_2_6_27;
> +	else
> +		page_offset = PAGE_OFFSET;
> +
> +	return 0;
> +}

Thanks,
  Dave

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