Re: Essay on an important Xen decision (long)

[Gerd Hoffmann <kraxel@suse.de>]

   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@suse.de>
I'm the hacker formerly known as Gerd Knorr.