Re: [Xen-devel] USB virt status --- Help please!!!

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 14 Nov 2005, at 21:46, Muli Ben-Yehuda wrote:

> > The alloc_skb is just avoiding one special case of this.  It's an
> > important special case, sure, but to be robust, might we not want to
> > have a minimal swiotlb cache available at all times as fallback?
> I think we certainly do. swiotlb will already avoid bouncing where
> possible via the checks for address_needs_mapping(). Is swiotlb
> prohibitively expensive even when no bouncing is necessary? if yes, we
> could either trim it down or just use it selectively as I outlined
> earlier on a per bus / device / subsystem basis.
Not expensive in time, but it does waste memory if it's unused. 
Currently we allocate a 64MB aperture if the machine has memory mapped 
above 2GB (this catches devices like aacraid that can only address 31 
bits). I could also allocate an aperture of, say, 2MB for smaller 
systems. There is of course a tension between not wasting memory yet 
having a big enough aperture that the system will not run out of iommu 
space and crash even when stressed.
 -- Keir


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