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

[Joerg Roedel]

On Tue, Sep 30, 2008 at 10:44:01PM +0300, Muli Ben-Yehuda wrote:
> 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.

This solves the problem with the DMA masks. But what happens to requests
that cross guest page boundarys?

> > 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. 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.

Joerg

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