RE: [Xen-devel] Question regarding behavior of virt_to_bus ....

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> 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). 

If it needs to be machine contiguous this won't work on its own.

You need to use xen_create_contiguous_region to attempt to change the
pages backing the region into a contigous extent. 

See dma_alloc

Ian

> 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
> --------------------------------------------------------------
> -----------
> 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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