Messages in this thread | | | Subject | Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct device | From | Alexey Kardashevskiy <> | Date | Mon, 6 Apr 2020 23:25:09 +1000 |
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On 06/04/2020 21:50, Christoph Hellwig wrote: > On Fri, Apr 03, 2020 at 07:38:11PM +1100, Alexey Kardashevskiy wrote: >> >> >> 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! > > Do you have a link to the whole branch? The github UI is unfortunately > unusable for that (or I'm missing something).
The UI shows the branch but since I rebased and forcepushed it, it does not. Here is the current one with:
https://github.com/aik/linux/commits/dma-bypass.3
Thanks,
-- Alexey
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