[Xen-devel] comment request: dom0 dma on large memory systems

["Scott Parish" <srparish@xxxxxxxxxx>]

purpose:

The patches i'm working on is taking more work then i was expecting; this
email's purpose is to get confirmation that the solution i'm working on
is the preferred one.


problem:

On x86_64 with 6gig ram, dom0's initial allocation is from memory
above the pci hole (referred to as "high memory" in this email) if
dom0_mem is set to 2g or higher. The only problem is that most io/dma
devices (non-dac) can only dma to the first 32bits worth of machine
addresses--thus for some configurations, dom0 has no memory which is
dma-able.

This is realized, in my experience, with dom0 unable to find any
partitions on scan and panicking because it can't mount root.


proposed solution to xen:

The attached patch is a (non-functional) prototype of my proposed
solution to the xen. The idea is to add a third MEMZONE for
dma'able memory, change the alloc_domheap_pages() interface so the
caller can request only dma memory if needed. Finally, internal to
common/page_alloc.c, split memory between MEMZONE_DOM and MEMZONE_DMADOM
based on the address.


proposed solution to dom0:

Dom0 might have memory allocated to it from both high and low memory.
The easiest solution would probably be to scan for and preallocate a
chunk of memory that will work for dma, something like what pci-gart.c
does. (or work on letting pci-gart.c work under xen)


sRp

-- 
Scott Parish
Signed-off-by: srparish@xxxxxxxxxx

Attachment: memzone-dmadom.diff
Description: Text document

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