Re: [Xen-devel] Re: [Patch] Constrain DMADOM/swiotlb allocations to 1G phys to fix b44 NIC driver

[Alan <alan@xxxxxxxxxxxxxxxxxxx>]

> > constraint for the DMADOM heap; and changes the x86/x86_64 swiotlb to
> > request a 30-bit constraint for allocations of the swiotlb memory.

(The hardware pseudo-iommu on the AMD can't handle this either as it can
only appear in the PCI window)

> The 30 bit constraint is enough to let b44 work.  But there are a number
> of sound cards which require a 28-bit DMA mask (256MB max physical
> addressable memory).  1GB seems reasonably safe, but I'm much less
> comfortable with restricting DMADOM to 256MB by default.  Does anyone
> have any strong feelings about shifting this?  What other users of
> DMADOM are there that might overflow if we restrict it even further?

For modern hardware the constraints are primarily the 30/31bit ones found
on some devices that know too much about Windows. The older sound cards
with 28bit limits like the ESS Maestro series are very much "old tech"
now. 

Alan

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