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?
