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xen-devel
Re: [Xen-devel] Essay on an important Xen decision (long)
Hi,
If the only issue for making dom0 VP is DMA, wouldn't it be easier to
modify the Linux DMA subsystem[1] to make a special hypercall to
essentially pin a VP to a particular MFN that could be used for the
DMA?
Linux has a nice API for DMA memory management, see
Documentation/DMA-mapping.txt. Basically you pass in a "struct page"
and a offset (within that page) and get back a dma address you can pass
on to your hardware. That is required for some architectures where
phyical addresses (as seen by the CPU) and bus addresses (as seen by the
pci devices) are not identical. It's also needed on archs which have an
iommu to create/delete mapping entries there.
I think that API should do just fine for any DMA transfer dom0 wants to
do for its own pages. xenlinux would simply need a special
implementation of that API which calls xen to translate the VP address
into a dma address (usually same as machine address). Probably xen must
also handle a iommu (if present) to ensure secure dma once we have
hardware which supports this.
A bit more tricky are DMA transfers for _other_ domains (i.e. what the
blkback driver has to do). blkback maps the foreign domain pages into
its own address space, and I think there is no way around that right now
API-wise as otherwise there isn't a "struct page" for the page ...
cheers,
Gerd
--
Gerd 'just married' Hoffmann <kraxel@xxxxxxx>
I'm the hacker formerly known as Gerd Knorr.
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