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

From: Koenig, Christian
Date: Wed Sep 04 2019 - 07:10:40 EST

Am 04.09.19 um 10:19 schrieb Thomas HellstrÃm (VMware):
> Hi, Christian,
> On 9/4/19 9:33 AM, Koenig, Christian wrote:
>> Am 03.09.19 um 23:05 schrieb Thomas HellstrÃm (VMware):
>>> On 9/3/19 10:51 PM, Dave Hansen wrote:
>>>> On 9/3/19 1:36 PM, Thomas HellstrÃm (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()?

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.


> 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.