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xen-devel
RE: [Xen-devel] comment request: dom0 dma on large memory systems
Aside from following question (just about the concept), actually to have
DMA sense within XEN is really necessary especially for a driver domain
N which has direct physical device access. Currently that domain N
invokes generic increase_reservation interface to get machine contiguous
pages. However that generic interface has no sense about the 4G
limitation for old DMA controller on 64bit platform. So such patch is
really necessary not only for domain0, but also all domains which has
direct relationship with physical devices. Also that hypercall interface
may also be changed a bit to contain flag whether DMA-able.
Thanks,
Kevin
>-----Original Message-----
>From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
>[mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Tian, Kevin
>Sent: Saturday, June 04, 2005 11:48 AM
>To: Scott Parish; xen-devel@xxxxxxxxxxxxxxxxxxx
>Subject: RE: [Xen-devel] comment request: dom0 dma on large memory
>systems
>
>----Original Message-----
>>From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
>>[mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Scott
>Parish
>>Sent: Friday, June 03, 2005 3:35 PM
>>
>>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.
>
>IIRC, 2 or 3 months ago, Keir said that default memory allocation for
>Dom0 is all available memory. And then CP has to decrease by balloon
>interface before creating other domains. If this still holds true, I'm
>not sure whether above problem still exists, since all avail memory
>including both <4G and >4G belonging to Dom0 then. (XEN itself only
>consumes a small trunk). However after looking at your patch and then
>the source, it seems that only the max available order, meaning must be
>continuous, is allocated to Dom0 currently. So did I misunderstand this
>concept? If it really only means maximum continuous trunk, then you
>patch definitely shoots straight on the real problem on all 64bit
>platform. ;-)
>
>Thanks,
>Kevin
>
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