Re: [PATCH v2 1/1] fs/splice: add missing callback for inaccessible pages

[Christian Borntraeger]

On 05.05.20 16:24, Dave Hansen wrote:
> On 5/5/20 7:00 AM, Christian Borntraeger wrote:
>> We are certainly not married to our approach. I would happily extend/change
>> this to anything that works for your case and the s390 case. So can you outline
>> your requirements a bit more?
> 
> For SEV, the guest define which pages are encrypted or not.  You could
> theoretically do DMA to them or have the CPU access their contents, but
> you'd get either get ciphertext for reads, or data corruption and loss
> of cache coherency for writes.  That's not so cool.
> 
> Ideally, we would stop the CPU from ever accessing those pages by
> unmapping them.  But, the pages go in and out of the encrypted state and
> the host really needs to be *sure* about what's going on before it
> restores its mapping and messes with the page.  That includes situations
> where someone does a gup, starts an I/O to an unencrypted page, then the
> guest tries to convert that page over to being encrypted.
> 
> 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?