[RFC PATCH] drm/ttm, drm/vmwgfx: Have TTM support AMD SEV encryption

[Koenig, Christian]

Am 24.05.19 um 11:55 schrieb Thomas Hellstrom:
> [CAUTION: External Email]
>
> On 5/24/19 11:11 AM, Thomas Hellstrom wrote:
>> Hi, Christian,
>>
>> On 5/24/19 10:37 AM, Koenig, Christian wrote:
>>> Am 24.05.19 um 10:11 schrieb Thomas Hellström (VMware):
>>>> [CAUTION: External Email]
>>>>
>>>> From: Thomas Hellstrom <thellstrom at vmware.com>
>>>>
>>>> With SEV encryption, all DMA memory must be marked decrypted
>>>> (AKA "shared") for devices to be able to read it. In the future we
>>>> might
>>>> want to be able to switch normal (encrypted) memory to decrypted in
>>>> exactly
>>>> the same way as we handle caching states, and that would require
>>>> additional
>>>> memory pools. But for now, rely on memory allocated with
>>>> dma_alloc_coherent() which is already decrypted with SEV enabled.
>>>> Set up
>>>> the page protection accordingly. Drivers must detect SEV enabled and
>>>> switch
>>>> to the dma page pool.
>>>>
>>>> This patch has not yet been tested. As a follow-up, we might want to
>>>> cache decrypted pages in the dma page pool regardless of their caching
>>>> state.
>>> This patch is unnecessary, SEV support already works fine with at least
>>> amdgpu and I would expect that it also works with other drivers as 
>>> well.
>>>
>>> Also see this patch:
>>>
>>> commit 64e1f830ea5b3516a4256ed1c504a265d7f2a65c
>>> Author: Christian König <christian.koenig at amd.com>
>>> Date:   Wed Mar 13 10:11:19 2019 +0100
>>>
>>>       drm: fallback to dma_alloc_coherent when memory encryption is
>>> active
>>>
>>>       We can't just map any randome page we get when memory
>>> encryption is
>>>       active.
>>>
>>>       Signed-off-by: Christian König <christian.koenig at amd.com>
>>>       Acked-by: Alex Deucher <alexander.deucher at amd.com>
>>>       Link: https://patchwork.kernel.org/patch/10850833/
>>>
>>> Regards,
>>> Christian.
>>
>> Yes, I noticed that. Although I fail to see where we automagically
>> clear the PTE encrypted bit when mapping coherent memory? For the
>> linear kernel map, that's done within dma_alloc_coherent() but for
>> kernel vmaps and and user-space maps? Is that done automatically by
>> the x86 platform layer?

Yes, I think so. Haven't looked to closely at this either.

>>
>> /Thomas
>>
> And, as a follow up question, why do we need dma_alloc_coherent() when
> using SME? I thought the hardware performs the decryption when DMA-ing
> to / from an encrypted page with SME, but not with SEV?

I think the issue was that the DMA API would try to use a bounce buffer 
in this case.

Christian.

>
> Thanks, Thomas
>
>
>