Re: [PATCH v2 3/4] drm/ttm, drm/vmwgfx: Correctly support support AMD memory encryption

[Christoph Hellwig <hch@infradead.org>]

On Wed, Sep 04, 2019 at 08:49:03AM +0200, Thomas Hellström (VMware) wrote:
> For device DMA address purposes, the encryption status is encoded in the dma
> address by the dma layer in phys_to_dma().
> 
> 
> >   There doesn’t seem to be any real funny business in dma_mmap_attrs() or dma_common_mmap().
> 
> No, from what I can tell the call in these functions to dma_pgprot()
> generates an incorrect page protection since it doesn't take unencrypted
> coherent memory into account. I don't think anybody has used these functions
> yet with SEV.

Yes, I think dma_pgprot is not correct for SEV.  Right now that function
isn't used much on x86, it had more grave bugs up to a few -rcs ago..

> > Would it make sense to add a vmf_insert_dma_page() to directly do exactly what you’re trying to do?
> 
> Yes, but as a longer term solution I would prefer a general dma_pgprot()
> exported, so that we could, in a dma-compliant way, use coherent pages with
> other apis, like kmap_atomic_prot() and vmap(). That is, basically split
> coherent page allocation in two steps: Allocation and mapping.

The thing is that dma_pgprot is of no help for you at all, as the DMA
API hides the page from you entirely.  In fact we do have backends that
do not even have a page backing.  But I think we can have a
vmf_insert_page equivalent that does the right thing behind your back
for the varius different implementation (contiguous page(s) in the kernel
lineary, contiguous page(s) with a vmap/ioremap remapping in various
flavours, non-contigous pages(s) with a vmap remapping, and deeply
magic firmware populated pools (well, except maybe for the last, but
at least we can fail gracefully there)).