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


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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