Re: [Xen-devel] [PATCH V2] arm: xen: mm: use __GPF_DMA32 for arm64

[Christoph Hellwig <hch@xxxxxxxxxxxxx>]

Can we take a step back and figure out what we want to do here?

AFAICS this function allocates memory for the swiotlb-xen buffer,
and that means it must be <= 32-bit addressable to satisfy the DMA API
guarantees.  That means we generally want to use GFP_DMA32 everywhere
that exists, but on systems with odd zones we might want to dip into
GFP_DMA.  This also means swiotlb-xen doesn't actually do the right
thing on x86 at the moment.  So shouldn't we just have one common
routine in swiotlb-xen.c that checks if we have CONFIG_ZONE_DMA32
set, then try GFP_DMA32, and if not check if CONFIG_ZONE_DMA is set
and then try that, else default to GFP_KERNEL?

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