Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct device

On 25/03/2020 19:37, Christoph Hellwig wrote:

On Wed, Mar 25, 2020 at 03:51:36PM +1100, Alexey Kardashevskiy wrote:

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This is for persistent memory which you can DMA to/from but yet it does
not appear in the system as a normal memory and therefore requires
special handling anyway (O_DIRECT or DAX, I do not know the exact
mechanics). All other devices in the system should just run as usual,
i.e. use 1:1 mapping if possible.

On other systems (x86 and arm) pmem as long as it is page backed does
not require any special handling.  This must be some weird way powerpc
fucked up again, and I suspect you'll have to suffer from it.

It does not matter if it is backed by pages or not, the problem may also
appear if we wanted for example p2p PCI via IOMMU (between PHBs) and
MMIO might be mapped way too high in the system address space and make
1:1 impossible.

How can it be mapped too high for a direct mapping with a 64-bit DMA
mask?

The window size is limited and often it is not even sparse. It requires
an 8 byte entry per an IOMMU page (which is most commonly is 64k max) so
1TB limit (a guest RAM size) is a quite real thing. MMIO is mapped to
guest physical address space outside of this 1TB (on PPC).


-- 
Alexey