Re: [RFC PATCH 07/35] Make LOAD_OFFSET defined by subarch

Dan Hecht
2006-03-22T22:57:55Z

Chris Wright wrote:
> Change LOAD_OFFSET so that the kernel has virtual addresses in the elf header fields.
>
> Unlike bare metal kernels, Xen kernels start with virtual address
> management turned on and thus the addresses to load to should be
> virtual addresses.
>

Rather than changing LOAD_OFFSET in Linux, why not leave this alone and
change the Xen domain builder to properly interpret the ELF program
header fields?

i.e. with this change, we'd have

p_paddr = __PAGE_OFFSET + segment_offset
p_vaddr = __PAGE_OFFSET + segment_offset
VIRT_BASE = __PAGE_OFFSET

where, the VA mapping p_paddr -> (p_paddr-VIRT_BASE) is established by
the domain builder.

Instead, why not drop this patch, and the VIRT_BASE portion of the
__xen_guest section, and instead change Xen's domain builder to treat
p_paddr and p_vaddr in a more standard way? Since Xen starts the domain
with virtual address management enabled, it makes sense for it to use
p_vaddr to determine the virtual address to load the kernel to. Then,
p_paddr could be used to determine which pseudo-physical pages back that
virtual address range.

i.e. use, just like vanilla linux:

p_paddr = segment_offset
p_vaddr = __PAGE_OFFSET + segment_offset

so these two fields directly indicate the same mapping as before, but
now in terms of p_vaddr -> p_paddr, which makes sense, and no need for
the extra VIRT_BASE attribute in the __xen_guest section.

Dan