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

[Christoph Hellwig <hch@infradead.org>]

On Fri, Aug 30, 2019 at 07:40:42PM -0700, Stefano Stabellini wrote:
> + 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.

But is it actually harmful?  If the GFP_DMA32 doesn't hurt we could
just use it there.  Or if that seems to ugly we can make the dma
flags dependents on a XEN_1TO1_MAPPED config option set by arm/arm64.

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