Messages in this thread | | | Date | Fri, 3 Oct 2008 11:38:20 +0300 | From | Muli Ben-Yehuda <> | Subject | Re: [PATCH 9/9] x86/iommu: use dma_ops_list in get_dma_ops |
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On Wed, Oct 01, 2008 at 09:19:56AM +0200, Joerg Roedel wrote:
> > It might be possible to have a per-device slow or fast path, where > > the fast path is for devices which have no DMA limitations > > (high-end devices generally don't) and the slow path is for > > devices which do. > > This solves the problem with the DMA masks. But what happens to > requests that cross guest page boundarys?
I'm not sure I follow. If a buffer is contiguous in the guest space, it will remain contiguous (i.e., be mapped contiguously) in the IOMMU I/O address space, even if each I/O PTE ends up mapping a different physical frame.
> > > With mapping/unmapping through hypercalls we add the > > > world-switch overhead to the copy-overhead. We can't avoid this > > > when we have no hardware support at all. But already with older > > > IOMMUs like Calgary and GART we can at least avoid the > > > world-switch. And since, for example, every 64 bit capable AMD > > > processor has a GART we can make use of it. > > > > It should be possible to reduce the number and overhead of > > hypercalls to the point where their cost is immaterial. I think > > that's fundamentally a better approach. > > Ok, we can queue map_sg allocations together an queue them into one > hypercall. But I remember a paper from you where you wrote that most > allocations are mapping only one area.
I'm afraid that bit of the paper was poorly done (mea culpa). As far as I can recall, the majority of dma_alloc_coherent + scatter-gather list *element* mappings only map a single frame, but we didn't look at the time at the average length of a scatter gather list and the frequency of sg list mappings vs. single page mappings. If the length and frequency are high enough, and you map entire sg lists in a single hcall or a single batch of hcalls, it might have a nice boost.
> Are there other ways to optimize this? I must say that reducing the > number of hypercalls was important while thinking about my idea. If > there are better ways I am all ears to hear from them.
There were a number of ideas mentioned in our paper (for example, switching drivers from the streaming DMA API to the persistent DMA API, which will be a big help to the scheme you propose), and Willman, Rixner and Cox also had some input to the problem[1]. Unfortunately no implementations exist yet AFAIK.
[1] "Protection Strategies for Direct Access to Virtualized I/O Devices", by Paul Willmann, Scott Rixner and Alan L. Cox, USENIX '08.
Cheers, Muli -- The First Workshop on I/O Virtualization (WIOV '08) Dec 2008, San Diego, CA, http://www.usenix.org/wiov08/ <-> SYSTOR 2009---The Israeli Experimental Systems Conference http://www.haifa.il.ibm.com/conferences/systor2009/
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