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

[Thomas HellstrÃm (VMware)]

On 9/3/19 10:51 PM, Dave Hansen wrote:
> On 9/3/19 1:36 PM, Thomas HellstrÃm (VMware) wrote:
> > So the question here should really be, can we determine already at mmap
> > time whether backing memory will be unencrypted and adjust the *real*
> > vma->vm_page_prot under the mmap_sem?
> >
> > Possibly, but that requires populating the buffer with memory at mmap
> > time rather than at first fault time.
> I'm not connecting the dots.
>
> vma->vm_page_prot is used to create a VMA's PTEs regardless of if they
> are created at mmap() or fault time.  If we establish a good
> vma->vm_page_prot, can't we just use it forever for demand faults?

With SEV I think that we could possibly establish the encryption flags 
at vma creation time. But thinking of it, it would actually break with 
SME where buffer content can be moved between encrypted system memory 
and unencrypted graphics card PCI memory behind user-space's back. That 
would imply killing all user-space encrypted PTEs and at fault time set 
up new ones pointing to unencrypted PCI memory..

> Or, are you concerned that if an attempt is made to demand-fault page
> that's incompatible with vma->vm_page_prot that we have to SEGV?
>
> > And it still requires knowledge whether the device DMA is always
> > unencrypted (or if SEV is active).
> I may be getting mixed up on MKTME (the Intel memory encryption) and
> SEV.  Is SEV supported on all memory types?  Page cache, hugetlbfs,
> anonymous?  Or just anonymous?

SEV AFAIK encrypts *all* memory except DMA memory. To do that it uses a 
SWIOTLB backed by unencrypted memory, and it also flips coherent DMA 
memory to unencrypted (which is a very slow operation and patch 4 deals 
with caching such memory).

/Thomas