Messages in this thread | | | Subject | Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct device | From | Alexey Kardashevskiy <> | Date | Fri, 3 Apr 2020 19:38:11 +1100 |
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On 26/03/2020 12:26, Alexey Kardashevskiy wrote: > > > On 25/03/2020 19:37, Christoph Hellwig wrote: >> On Wed, Mar 25, 2020 at 03:51:36PM +1100, Alexey Kardashevskiy wrote: >>>>> 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). > >
I am trying now this approach on top of yours "dma-bypass.3" (it is "wip", needs an upper boundary check):
https://github.com/aik/linux/commit/49d73c7771e3f6054804f6cfa80b4e320111662d
Do you see any serious problem with this approach? Thanks!
-- Alexey
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