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

[Alex Deucher]

+ Tom

He's been looking into SEV as well.

On Fri, May 24, 2019 at 8:30 AM Thomas Hellstrom <thomas at shipmail.org> wrote:
>
> On 5/24/19 2:03 PM, Koenig, Christian wrote:
> > Am 24.05.19 um 12:37 schrieb Thomas Hellstrom:
> >> [CAUTION: External Email]
> >>
> >> On 5/24/19 12:18 PM, Koenig, Christian wrote:
> >>> 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.
> >> This sounds a bit odd. If that were the case, the natural place would be
> >> the PAT tracking code, but it only handles caching flags AFAICT. Not
> >> encryption flags.
> >>
> >> But when you tested AMD with SEV, was that running as hypervisor rather
> >> than a guest, or did you run an SEV guest with PCI passthrough to the
> >> AMD device?
> > Yeah, well the problem is we never tested this ourself :)
> >
> >>>>> /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.
> >> SEV forces SWIOTLB bouncing on, but not SME. So it should probably be
> >> possible to avoid dma_alloc_coherent() in the SME case.
> > In this case I don't have an explanation for this.
> >
> > For the background what happened is that we got reports that SVE/SME
> > doesn't work with amdgpu. So we told the people to try using the
> > dma_alloc_coherent() path and that worked fine. Because of this we came
> > up with the patch I noted earlier.
> >
> > I can confirm that it indeed works now for a couple of users, but we
> > still don't have a test system for this in our team.
> >
> > Christian.
>
> OK, undestood,
>
> But unless there is some strange magic going on, (which there might be
> of course),I do think the patch I sent is correct, and the reason that
> SEV works is that the AMD card is used by the hypervisor and not the
> guest, and TTM is actually incorrectly creating conflicting maps and
> treating the coherent memory as encrypted. But since the memory is only
> accessed through encrypted PTEs, the hardware does the right thing,
> using the hypervisor key for decryption....
>
> But that's only a guess, and this is not super-urgent. I will be able to
> follow up if / when we bring vmwgfx up for SEV.
>
> /Thomas
>
> >> /Thomas
> >>
> >>
> >>> Christian.
> >>>
> >>>> Thanks, Thomas
> >>>>
> >>>>
> >>>>
>
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