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Subject: Re: [PATCH] lib/scatterlist: Provide a DMA page iterator
To:     Jason Gunthorpe <jgg@ziepe.ca>,
        Thomas Hellstrom <thellstrom@vmware.com>
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From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <ckoenig.leichtzumerken@gmail.com>
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Date:   Wed, 16 Jan 2019 11:40:48 +0100
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Am 15.01.19 um 22:25 schrieb Jason Gunthorpe:
> On Tue, Jan 15, 2019 at 02:17:26PM +0000, Thomas Hellstrom wrote:
>> Hi, Christoph,
>>
>> On Mon, 2019-01-14 at 10:48 +0100, Christoph Hellwig wrote:
>>> On Thu, Jan 10, 2019 at 04:42:18PM -0700, Jason Gunthorpe wrote:
>>>>> Changes since the RFC:
>>>>> - Rework vmwgfx too [CH]
>>>>> - Use a distinct type for the DMA page iterator [CH]
>>>>> - Do not have a #ifdef [CH]
>>>> ChristophH: Will you ack?
>>> This looks generally fine.
>>>
>>>> Are you still OK with the vmwgfx reworking, or should we go back to
>>>> the original version that didn't have the type safety so this
>>>> driver
>>>> can be left broken?
>>> I think the map method in vmgfx that just does virt_to_phys is
>>> pretty broken.  Thomas, can you check if you see any performance
>>> difference with just doing the proper dma mapping, as that gets the
>>> driver out of interface abuse land?
>> The performance difference is not really the main problem here. The
>> problem is that even though we utilize the streaming DMA interface, we
>> use it only since we have to for DMA-Remapping and assume that the
>> memory is coherent. To be able to be as compliant as possible and ditch
>> the virt-to-phys mode, we *need* a DMA interface flag that tells us
>> when the dma_sync_for_xxx are no-ops. If they aren't we'll refuse to
>> load for now. I'm not sure, but I think also nouveau and radeon suffer
>> from the same issue.
> RDMA needs something similar as well, in this case drivers take a
> struct page * from get_user_pages() and need to have the DMA map fail
> if the platform can't DMA map in a way that does not require any
> additional DMA API calls to ensure coherence. (think Userspace RDMA
> MR's)
>
> Today we just do the normal DMA map and when it randomly doesn't work
> and corrupts data tell those people their platforms don't support RDMA
> - it would be nice to have a safer API base solution..

Oh, yes really good point. We have to support get_user_pages (or HMM) in 
a similar manner on GPUs as well.

Regards,
Christian.

>
> Jason
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