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xen-devel
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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