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

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> Since Andi's ZONE_DMA32 patches went in last night, how about 
> foregoing the up-front allocation of bounce buffers and 
> allocating them as necessary from ZONE_DMA32? once we have 
> that, we should be able to switch to swiotlb by default.

It's not quite that simple: We need memory that is machine contiguous,
which means we need to call into Xen with xen_alloc_contig.

Xen is unlikely to have any memory to allocate that is machine
contiguous to the current slab, which means we'd have to introduce
swiotlb to the notion of having multiple apertures. This wouldn't be a
bad thing, but would complicate the code.

Having multiple apertures does rather increase the chances that we'd
avoid the fatal problem of dma_alloc_single not being able to return
failure.

Ian

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