linux-kernel - Re: [PATCH 1/2] dma-mapping: add a dma_ops

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Date:   Mon, 6 Apr 2020 23:25:09 +1000
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Christoph Hellwig <hch@....de>
Cc:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        iommu@...ts.linux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        linux-kernel@...r.kernel.org, Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct
 device



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