On 9/19/23 07:39, Christian König wrote:
Am 19.09.23 um 03:26 schrieb Zack Rusin:
On Mon, 2023-09-18 at 16:21 -0400, Alex Deucher wrote:
!! External Email
On Mon, Sep 18, 2023 at 3:06 PM Thomas Hellström
<thomas.hellstrom@xxxxxxxxxxxxxxx> wrote:
On 9/18/23 17:52, Zack Rusin wrote:
On Mon, 2023-09-18 at 17:13 +0200, Thomas Hellström wrote:
Hi,
On 9/18/23 16:56, Thomas Hellström wrote:
Hi Zack, Christian
On 9/18/23 13:36, Christian König wrote:
Hi Zack,
adding Thomas and Daniel.
I briefly remember that I talked with Thomas and some other people
about that quite a while ago as well, but I don't fully
remember the
outcome.
Found one old thread, but didn't read it:
https://lists.freedesktop.org/archives/dri-devel/2019-September/234100.html
/Thomas
Ugh. Now starting to read that thread I have a vague recollection
it all
ended with not supporting mapping any device pages whatsoever
when SEV
was enabled, but rather resorting to llvmpipe and VM-local bos.
Hi, Thomas.
Thanks for finding this! I'd (of course) like to solve it properly
and get
vmwgfx
running with 3d support with SEV-ES active instead of essentially
disabling
the
driver when SEV-ES is active.
I think there are two separate discussions there, the
non-controversial one
and the
controversial one:
1) The non-controversial: is there a case where drivers would want
encrypted
memory
for TT pages but not for io mem mappings? Because if not then as
Christian
pointed
out we could just add pgprot_decrypted to ttm_io_prot and be
essentially done.
The
current method of decrypting io mem but leaving sys mem mappings
encrypted is
a bit
weird anyway.
If the answer to that question is "yes, some driver does want the
TT mappings
to be
encrypted" then your "[PATCH v2 3/4] drm/ttm, drm/vmwgfx:
Correctly support
support
AMD memory encryption" solves that. I think getting one of those
two in makes
sense
regardless of everything else, agreed?
Well, there is more to it I think.
IIRC, the AMD SME encryption mode has a way for a device to have the
memory controller (?) encrypt / decrypt device traffic by using an
address range alias, so in theory it supports encrypted TT pages, and
the dma-layer may indeed hand encrypted DMA pages to TTM on such
systems
depending on the device's DMA mask. That's why I think that
force_dma_unencrypted() export was needed, and If the amdgpu driver
accesses TT memory in SME mode *without* pgprot_decrypted() and it
still
works, then I think that mode is actually used. How could it
otherwise work?
For SME, as long as the encrypted bit is set in the physical address
used for DMA, the memory controller will handle the encrypt/decrypt
for the device. For devices with a limited dma mask, you need to use
the IOMMU so that the encrypted bit is retained when the address hits
the memory controller.
How does that work on systems with swiotlb, e.g. swiotlb=force, or
i.e. what would
decrypt the ttm tt mappings when copying between system and vram when
iommu is
disabled/absent?
SME makes it mandatory that all devices can handle the physical
address used for DMA, either native or with the help of IOMMU.
Hacks like SWIOTLB are not directly supported as far as I know. Maybe
somehow SWIOTLB manually decrypts the data while copying it or
something like this, but I'm not 100% sure if that is actually
implemented.
Regards,
Christian.
A bold guess after looking at various code and patches:
1) Devices under SME that don't support the encryption bit and SEV:
a) Coherent memory is unencrypted.
b) Streaming DMA under IOMMU: The IOMMU sets the encrypted bit.
c) Streaming DMA with SWIOTLB: The bounce buffer is unencrypted. Copying
to/from bounce-buffer decrypts/encrypts.
2) Devices under SME that do support the encryption bit (which I believe
is most graphics devices in general on SME systems, not just amdgpu; it
"just works")
*) Coherent memory is encrypted. The DMA layer sets dma addresses and
pgprot accordingly.
*) Streaming DMA is encrypted.
So the bug in TTM would then be it's not handling 1a) and 1b) correctly.
Remedy:
1b) Shouldn't be used with encryption.
1a) This is what we should try to fix. Exporting dma_force_unencrypted()
didn't seem to be a way forward. Properly fixing this would, I guess,
mean implement the missing functionality in the dma layer: For vmap /
kmap we could simply reuse the virtual addresses we get back from
dma_alloc_coherent(), but for faulting one would want something like
dma_coherent_insert_pfn() (if it doesn't exist already) after a proper
disussion with Christoph Hellwig.
/Thomas
z
