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

From: Thomas HellstrÃm (VMware)
Date: Tue Sep 03 2019 - 16:36:35 EST

On 9/3/19 9:55 PM, Dave Hansen wrote:
On 9/3/19 12:51 PM, Daniel Vetter wrote:
The thing we need to stop is having mixed encryption rules under one VMA.
The point here is that we want this. We need to be able to move the
buffer between device ptes and system memory ptes, transparently,
behind userspace back, without races. And the fast path (which is "no
pte exists for this vma") must be real fast, so taking mmap_sem and
replacing the vma is no-go.
So, when the user asks for encryption and we say, "sure, we'll encrypt
that", then we want the device driver to be able to transparently undo
that encryption under the covers for device memory? That seems suboptimal.

I'd rather the device driver just say: "Nope, you can't encrypt my VMA".
Because that's the truth.

The thing here is that it's the underlying physical memory that define the correct encryption flags. If it's DMA memory and SEV is active or PCI memory. It's always unencrypted. User-space in a SEV vm should always, from a data protection point of view, *assume* that graphics buffers are unencrypted. (Which will of course limit the use of gpus and display controllers in a SEV vm). Platform code sets the vma encryption to on by default.

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.

And it still requires knowledge whether the device DMA is always unencrypted (or if SEV is active).