Re: Essay on an important Xen decision (long)

[Anthony Liguori <aliguori@us.ibm.com>]

Gerd Hoffmann wrote:

>   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.

Excellent, thanks for the reference!

>
> 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 ...

There are, of course, other ways around this.  One could have a 
hypervisor level DMA API that allowed bulk transfer of memory between 
domains (either by copying or page flipping depending the size of the 
buffer).  Another option would be a separate pool of sharable memory 
that could be mapped appropriately into a domain's VPM space.

Regards,

Anthony Liguori

> cheers,
>
>   Gerd
>