[Xen-devel] Re: Questions about VIRT_BASE and ELF_PADDR_OFFSET in __xen_guest

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 18 May 2006, at 19:15, Puthiyaparambil, Aravindh wrote:

> I was looking at your changeset:
> http://xenbits.xensource.com/xen-unstable.hg?cs=dbe396fd6fd8
>
> I am finding that by making VIRT_BASE=0x0 is causing the builder to seg
> fault. The same thing happens when VIRT_BASE and ELF_PADDR_OFFSET is 
> set
> to 0. I don't understand why dsi->elf_paddr_offset is being set to
> VIRT_BASE and then overridden with ELF_PADDR_OFFSET if it is set. Does
> ELF_PADDR_OFFSET have any real use? Does it correlate with anything in
> the ELF program header? I am a little confused here :-), so could you
> please explain.
VIRT_BASE is the linear address you want your memory allocation mapped 
at.
ELF_PADDR_OFFSET is a value subtracted from addresses in the Elf 
headers to produce a pseudo-physical address. We usually set it to 
VIRT_BASE since we write virtual (linear) addresses into those Elf 
header fields. The default of setting it to VIRT_BASE is simply to 
maintain backward compatibility with old guest images which do not 
explicitly define it.
There is no reason really why VIRT_BASE=0 should not work. If it 
crashes there is presumably some underlying bug which disallowing 
VIRT_BASE=0 does not fix.
 -- Keir


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