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

[Thomas Hellström (VMware)]

On 9/4/19 1:10 PM, Koenig, Christian wrote:
> Am 04.09.19 um 10:19 schrieb Thomas Hellström (VMware):
>> Hi, Christian,
>>
>> On 9/4/19 9:33 AM, Koenig, Christian wrote:
>>> Am 03.09.19 um 23:05 schrieb Thomas Hellström (VMware):
>>>> 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..
>>> Well my problem is where do you see encrypted system memory here?
>>>
>>> At least for AMD GPUs all memory accessed must be unencrypted and that
>>> counts for both system as well as PCI memory.
>> We're talking SME now right?
>>
>> The current SME setup is that if a device's DMA mask says it's capable
>> of addressing the encryption bit, coherent memory will be encrypted.
>> The memory controllers will decrypt for the device on the fly.
>> Otherwise coherent memory will be decrypted.
>>
>>> So I don't get why we can't assume always unencrypted and keep it
>>> like that.
>> I see two reasons. First, it would break with a real device that
>> signals it's capable of addressing the encryption bit.
> Why? Because we don't use dma_mmap_coherent()?

Well, assuming always unencrypted would obviously break on a real device 
with encrypted coherent memory?

dma_mmap_coherent() would work from the encryption point of view 
(although I think it's currently buggy and will send out an RFC for what 
I believe is a fix for that).

>
> I've already talked with Christoph that we probably want to switch TTM
> over to using that instead to also get rid of the ttm_io_prot() hack.

OK, would that mean us ditching other memory modes completely? And 
on-the-fly caching transitions? or is it just for the special case of 
cached coherent memory? Do we need to cache the coherent kernel mappings 
in TTM as well, for ttm_bo_kmap()?

/Thomas

>
> Regards,
> Christian.
>
>> Second I can imagine unaccelerated setups (something like vkms using
>> prime feeding a VNC connection) where we actually want the TTM buffers
>> encrypted to protect data.
>>
>> But at least the latter reason is way far out in the future.
>>
>> So for me I'm ok with that if that works for you?
>>
>> /Thomas
>>
>>
>>> Regards,
>>> Christian.
>>