Re: [PATCH 9/9] x86/iommu: use dma_ops_list in get_dma_ops

[Muli Ben-Yehuda <muli@il.ibm.com>]

On Fri, Sep 26, 2008 at 02:32:43PM +0200, Joerg Roedel wrote:

> Ok, the allocation only matters for dma_alloc_coherent. Fujita
> introduced a generic software-based dma_alloc_coherent recently
> which you can use for that. I think implementing PVDMA into an own
> dma_ops backend and multiplex it using my patches introduces less
> overhead than an additional layer over the current dma_ops
> implementation.

I'm not sure what you have in mind, but I agree with Amit that
conceptually pvdma should be called after the guest's "native" dma_ops
have done their thing. This is not just for nommu, consider a guest
that is using an (emulated) hardware IOMMU, or that wants to use
swiotlb. We can't replicate their functionality in the pv_dma_ops
layer, we have to let them run first and then pass deal with whatever
we get back.

> Another two questions to your approach: What happens if a
> dma_alloc_coherent allocation crosses page boundarys and the gpa's
> are not contiguous in host memory? How will dma masks be handled?

That's a very good question. The host will need to be aware of a
device's DMA capabilities in order to return I/O addresses (which
could be hpa's if you don't have an IOMMU) that satisfy them. That's
quite a pain.

Cheers,
Muli
-- 
The First Workshop on I/O Virtualization (WIOV '08)
Dec 2008, San Diego, CA, http://www.usenix.org/wiov08/
                      xxx
SYSTOR 2009---The Israeli Experimental Systems Conference
http://www.haifa.il.ibm.com/conferences/systor2009/