Xen devel - Oct 2005 - Question regarding behavior of virt_to_bus ....

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
-------------------------------------------------------------------------
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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> 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.
> --------------------------------------------------------------
> -----------
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel
> 
_______________________________________________
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But I do call xen_create_contiguous_region.

On Tue, Oct 04, 2005 at 11:10:21AM +0100, Ian Pratt
wrote:
> > 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.
> > --------------------------------------------------------------
> > -----------
> > 
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@lists.xensource.com
> > http://lists.xensource.com/xen-devel
> > 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel
-- 
-------------------------------------------------------------------------
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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