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

On Thu, Nov 22, 2018 at 08:50:47AM -0800, Linus Torvalds wrote:
> On Thu, Nov 22, 2018 at 6:03 AM Christoph Hellwig <hch@xxxxxx> wrote:
> >
> > The 0 return doesn't work for direct mappings that have ram at address
> > zero and a lot of IOMMUs that start allocating bus space from address
> > zero, so we can't consolidate on that, but I think we can move everyone
> > to all-Fs, which the patch here does.
> Hmm. Maybe not limit it to just all ones, but actually use the
> (standard, for the kernel) IS_ERR_VALUE()?
> That basically reserves the last 4095 values in an unsigned long for
> error values.
> Then those functions could actually return *what* error they
> encountered, using just plain
>         return -ENOMEM;
> or whatever?


I'm afraid that won't work very well - 32 bit platforms with 64-bit
addresses (LPAE) would have dma_addr_t as a 64-bit value, which
wouldn't fit into an unsigned long.

IS_ERR_VALUE() would cast a 64-bit DMA address down to a 32-bit
pointer (effectively masking with 0xffffffff).  It would have the
effect of making (eg) 0xXXXXXXXX_fffffVVV an error, where XXXXXXXX
are any of the top 32-bits of a 64-bit bus address, and VVV is the
error code value.

That could be a problem if you hit it in several places throughout
your available RAM... we'd have to mark every top page of RAM in
a naturally aligned 4GB as unusable, as well as block the top page
in natually aligned 4GB blocks from IOMMUs... and then what about
buses that have weird offsets...

So, I don't think the IS_ERR_VALUE() would work very well.

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