Re: [PATCH v3] drm/ttm: Make sure the mapped tt pages are decrypted when needed

[Zack Rusin]

On Fri, Jan 5, 2024 at 8:51 AM Zack Rusin <zack.rusin@xxxxxxxxxxxx> wrote:
>
> Some drivers require the mapped tt pages to be decrypted. In an ideal
> world this would have been handled by the dma layer, but the TTM page
> fault handling would have to be rewritten to able to do that.
>
> A side-effect of the TTM page fault handling is using a dma allocation
> per order (via ttm_pool_alloc_page) which makes it impossible to just
> trivially use dma_mmap_attrs. As a result ttm has to be very careful
> about trying to make its pgprot for the mapped tt pages match what
> the dma layer thinks it is. At the ttm layer it's possible to
> deduce the requirement to have tt pages decrypted by checking
> whether coherent dma allocations have been requested and the system
> is running with confidential computing technologies.
>
> This approach isn't ideal but keeping TTM matching DMAs expectations
> for the page properties is in general fragile, unfortunately proper
> fix would require a rewrite of TTM's page fault handling.
>
> Fixes vmwgfx with SEV enabled.
>
> v2: Explicitly include cc_platform.h
> v3: Use CC_ATTR_GUEST_MEM_ENCRYPT instead of CC_ATTR_MEM_ENCRYPT to
> limit the scope to guests and log when memory decryption is enabled.

Sorry, this also got a bit lost during the s/VMware/Broadcom/
transition. It seems to be pretty safe in general now. I wasn't able
to find a really clean way of adding a warn_once when pte's don't
match as suggested by Thomas, but I did add a quick log to at least
point out in the logs that we've enabled memory decryption in tt.

z