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Date:   Tue, 5 May 2020 07:34:39 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        Ulrich Weigand <uweigand@...ibm.com>
Cc:     Claudio Imbrenda <imbrenda@...ux.ibm.com>, viro@...iv.linux.org.uk,
        david@...hat.com, akpm@...ux-foundation.org, aarcange@...hat.com,
        linux-mm@...ck.org, frankja@...ux.ibm.com, sfr@...b.auug.org.au,
        jhubbard@...dia.com, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, jack@...e.cz, kirill@...temov.name,
        peterz@...radead.org, sean.j.christopherson@...el.com,
        Ulrich.Weigand@...ibm.com
Subject: Re: [PATCH v2 1/1] fs/splice: add missing callback for inaccessible
 pages

On 5/5/20 7:31 AM, Christian Borntraeger wrote:
>> So, the requirements are:
>>
>> 1. Allow host-side DMA and CPU access to shared pages
>> 2. Stop host-side DMA and CPU access to encrypted pages
>> 3. Allow pages to be converted between the states at the request of the
>>    guest
>>
>> Stopping the DMA is pretty easy, even across the gazillions of drivers
>> in the tree because even random ethernet drivers do stuff like:
>>
>>                 txdr->buffer_info[i].dma =
>>                         dma_map_single(&pdev->dev, skb->data, skb->len,
>>                                        DMA_TO_DEVICE);
>>
>> So the DMA can be stopped at the mapping layer.  It's a *LOT* easier to
>> catch there since the IOMMUs already provide isolation between the I/O
>> and CPU address spaces.
> And your problem is that the guest could convert this after the dma_map?
> So you looked into our code if this would help?

Yep, it seemed like a close-enough problem.

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