Question regarding behavior of virt_to_bus ....

[Himanshu Raj <rhim@cc.gatech.edu>]

Hi Folks,

I need some large chunk of physical memory, so I allocate it early using 
alloc_bootmem_low_pages and use it later (akin to bigphysarea patch). 
The trouble I am having is the following. Suppose the virtual address 
for the memory area is 0xc14d6000. After xen_create_contiguous_region 
call, I do a virt_to_bus and the bus address comes out to be 0x18000000. 
So far so good. However, later when I try to use this memory in a device 
driver, the call to virt_to_bus returns 0x14d6000. This result is 
baffling me, since that's what it would look like in a normal 32-bit 
i386 world, and not in xenified world.

Am I doing something stupid, or is there some technicality of memory 
subsystem that I don't get. Ofcourse, if I just use pci_alloc_consistent 
(for a small buffer), everything works fine, since there is no bootmem 
related contortion involved.

I will highly appreciate any input on this.

--Himanshu
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Himanshu Raj
PhD Student, GaTech (www.cc.gatech.edu/~rhim)
I prefer to receive attachments in an open, non-proprietary format.
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