Messages in this thread | | | Date | Tue, 30 Sep 2008 22:44:01 +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 Mon, Sep 29, 2008 at 03:33:11PM +0200, Joerg Roedel wrote:
> > Nobody cares about the performance of dma_alloc_coherent. Only the > > performance of map_single/map_sg matters. > > > > I'm not sure how expensive the hypercalls are, but they are more > > expensive than bounce buffering coping lots of data for every > > I/Os? > > I don't think that we can avoid bounce buffering into the guests at > all (with and without my idea of a paravirtualized IOMMU) when we > want to handle dma_masks and requests that cross guest physical > pages properly.
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.
> 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.
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/
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