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List:       linux-arm-kernel
Subject:    Re: [PATCH V2] arm: xen: mm: use __GPF_DMA32 for arm64
From:       Stefano Stabellini <sstabellini () kernel ! org>
Date:       2019-08-31 2:40:42
Message-ID: alpine.DEB.2.21.1908301926500.21347 () sstabellini-ThinkPad-T480s
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+ Juergen, Boris

On Fri, 30 Aug 2019, Christoph Hellwig wrote:
> 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?

Yes, for ARM/ARM64 it makes a lot of sense given that dom0 is 1:1 mapped
(pseudo-physical == physical).  I'll let Juergen and Boris comment on
the x86 side of things, but on x86 PV Dom0 is not 1:1 mapped so
GFP_DMA32 is probably not meaningful.

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