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

[Thomas HellstrÃm (VMware)]

On 9/4/19 1:10 PM, Koenig, Christian wrote:
> 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()?

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.