On Thu, Jan 23, 2014 at 02:15:07PM -0500, Nicolas Pitre wrote:

On Wed, 22 Jan 2014, Wang Nan wrote:

quoted

This patch allows the kernel to be loaded at the middle of kernel awared
physical memory. Before this patch, users must use mem= or device tree to cheat
kernel about the start address of physical memory.

This feature is useful in some special cases, for example, building a crash
dump kernel. Without it, kernel command line, atag and devicetree must be
adjusted carefully, sometimes is impossible.

With CONFIG_PATCH_PHYS_VIRT the value for PHYS_OFFSET is determined 
dynamically by rounding down the kernel image start address to the 
previous 16MB boundary.  In the case of a crash kernel, this might be 
cleaner to simply readjust __pv_phys_offset during early boot and call 
fixup_pv_table(), and then reserve away the memory from the previous 
kernel.  That will let you access that memory directly (with gdb for 
example) and no pointer address translation will be required.

We already have support in the kernel to ignore memory below the calculated
PHYS_OFFSET.  See 571b14375019c3a66ef70d4d4a7083f4238aca30.

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