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SubjectRe: [PATCH] s390 (5/7): kmalloc arguments.
Pete Zaitcev wrote:

>What does GFP_DMA do on s390 and s390x?

On s390, nothing.

On s390x, it makes sure the allocated memory resides at
addresses below 2 GB. This is necessary, as many of the
I/O subsystem data structures as defined by the hardware
contain pointer fields that are still 31-bit, even on
64-bit machines. Thus we have to make sure those data
structures are allocated below 2 GB. Using the GFP_DMA
mechanism for that purpose seemed to be the way to go ...

Note that contrary to the usual purpose of GFP_DMA on Intel,
the actual *data* that is being transferred via the I/O
subsystem can reside at arbitrary addresses (which are
specified via indirect-addressing lists); it is only the
control data structures that need to go below 2 GB.


Dr. Ulrich Weigand
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