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

[Thomas Hellstrom <thomas@shipmail.org>]

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@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@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@amd.com>
>>       Acked-by: Alex Deucher <alexander.deucher@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?
>
> /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?

Thanks, Thomas



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