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

[Andy Lutomirski <luto@kernel.org>]

On Tue, Sep 3, 2019 at 2:05 PM Thomas Hellström (VMware)
<thomas_os@shipmail.org> wrote:
>
> On 9/3/19 10:51 PM, Dave Hansen wrote:
> > On 9/3/19 1:36 PM, Thomas Hellström (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).
>

I'm still lost.  You have some fancy VMA where the backing pages
change behind the application's back.  This isn't particularly novel
-- plain old anonymous memory and plain old mapped files do this too.
Can't you all the insert_pfn APIs and call it a day?  What's so
special that you need all this magic?  ISTM you should be able to
allocate memory that's addressable by the device (dma_alloc_coherent()
or whatever) and then map it into user memory just like you'd map any
other page.

I feel like I'm missing something here.