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Re: [Xen-devel] remove the ->mapping_error method from dma_map_ops V2

On Wed, Nov 28, 2018 at 11:27:17AM -0800, David Miller wrote:
> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Date: Wed, 28 Nov 2018 10:00:06 -0800
> > Not all memory is accessible even to the kernel. If you have memory
> > that shows up in the last page of phys_addr_t, you just mark it
> > reserved at boot-time.
> It's not the physical memory at the end that needs to be reserved.
> It's the IOMMU mapping arena.

True, if and only if you have an IOMMU.

Where there isn't an IOMMU, then we'd have to reserve every page that
that translates to a bus address in the top 4K of dma_addr_t on any
bus in the system - that means knowing early in the kernel
initialisation about all buses in the system so we can detect and
reserve these pages.

I don't think that's trivial to do.

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