Decrypting tt maps in ttm

Thu Sep 21 03:51:19 UTC 2023

On Wed, 2023-09-20 at 21:22 +0200, Thomas Hellström wrote:
> !! External Email
>
> On 9/20/23 20:24, Zack Rusin wrote:
> > On Wed, 2023-09-20 at 19:17 +0200, Thomas Hellström wrote:
> > > !! External Email
> > >
> > > Hi, Zack
> > >
> > > On 9/20/23 18:39, Zack Rusin wrote:
> > > > On Wed, 2023-09-20 at 12:48 +0200, Christian König wrote:
> > > > > !! External Email
> > > > >
> > > > > Am 20.09.23 um 09:36 schrieb Thomas Hellström:
> > > > > > Hi, Zack,
> > > > > >
> > > > > > On 9/20/23 05:43, Zack Rusin wrote:
> > > > > > > On Tue, 2023-09-19 at 09:47 +0200, Christian König wrote:
> > > > > > > > !! External Email
> > > > > > > >
> > > > > > > > Am 19.09.23 um 08:56 schrieb Thomas Hellström:
> > > > > > > > > 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 at linux.intel.com> 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.
> > > > > > > > Christoph once pointed me to dma_mmap_attrs() for this, but I never
> > > > > > > > found the time to fully look into it.
> > > > > > > Hmm, yea, that would make sense
> > > > > > > https://elixir.bootlin.com/linux/latest/source/kernel/dma/direct.c#L564
> > > > > > > Replacing the vmap's with dma_mmap_attrs would probably fix this, but
> > > > > > > it would
> > > > > > > require a bit of extra setup.
> > > > > > >
> > > > > > > So we're saying that yes, we don't want unconditional pgprot_decrypt
> > > > > > > in ttm_io_prot.
> > > > > > > We'd like to leave those tt mappings as encrypted when possible and
> > > > > > > instead maybe
> > > > > > > add a vaddr to ttm_tt (or extract it from the pages->private via the
> > > > > > > ttm_pool_dma,
> > > > > > > but that seems rather ugly),
> > > > > > It could probably be extracted from pages->private from a helper in
> > > > > > the ttm pool code, (Christian has a final saying here). However, that
> > > > > > requires that all ttm_tts are built from a single dma_alloc chunk. Not
> > > > > > sure that's the case? In that case we're back to square zero for vmaps.
> > > > > Nope they aren't and yes we are back to square one with that.
> > > > Well, that's my favorite square. Number one, just like me...
> > > >
> > > > Maybe we're overthinking this particular problem a bit. As is use_dma_alloc
> > > > in
> > > > ttm
> > > > is only set in two cases:
> > > > - driver explicitly wants coherent mappings (vmwgfx, which require decrypted
> > > > pages)
> > > > - driver needs swiotlb (which, as was pointed out, would require the pages
> > > > to be
> > > > decrypted as well)
> > > >
> > > > So use_dma_alloc always requires the pages to be decrypted.
> > > IIRC moving forward it doesn't, since there is (or at least there was)
> > > implement missing TTM functionality in the dma layer and most TTM
> > > drivers should at least support dma coherent memory. That means all
> > > devices supporting a sufficiently large dma mask will break with SME and
> > > your proposal then.
> > >
> > > Perhaps if we condition that on
> > > "cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)" that will capture all the
> > > SEV cases, and limit the existing bug to the hopefully very few TTM
> > > devices with limited dma mask on SME.
> > Ah, I wasn't aware those exist, do you know what platforms are those? I can try
> > to
> > find one around here to see.
>
> My understanding is cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) will
> return true in the guest iff SEV is active, and can be used in TTM as a
> poor man's force_dma_unencrypted(), enabling the functionality in your
> diff. It looks like a similar check is present in vmwgfx to detect SEV,
> but also see below.
>
> > And they don't really break, they just might unnecessarily decrypt tt pages,
> > right?
>
> No, with SME, dma from hw will encrypt the content, because the dma
> layer will set the "encrypt" bit in the physical address given to the
> iommu or the device in case iommu is not active, but a subsequent
> reading the content using the CPU won't decrypt so CPU and device will
> have different views of the page.
>
> Also the linear kernel mapping PTEs will conflict in encryption mode
> with the ones TTM sets up, and IIRC that's forbidden in the SEV spec.
> (The x86 arch code goes through some serious work to flush out caches
> and TLBs to convert a page kernel linear mapping from encrypted to
> non-encrypted,
>
> https://elixir.bootlin.com/linux/latest/source/arch/x86/mm/pat/set_memory.c#L2129
>
> and that is also seen as pretty heavy dma_alloc_coherent() latency).

Thanks for this!

> So the pgprot_t TTM sets up *must* be identical to the one used by the
> dma layer, so anything we should be aware here that anything we do in
> TTM less than adding needed functionality in the dma layer is
> second-guessing what the dma layer does internally and is not really the
> right solution.

I think this is already the case for virtualized drivers, but I see what you're
saying that fixing this for them might break some real hardware and that's bad.
Playing those games with matching pgprot between ttm and dma is really fragile.

cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) is also used in drm_need_swiotlb so
adding that check to the last patch would seem to make sense. Of course, it's up to
Christian whether that's robust enough or whether we need to think about the
dma/page fault rework to fix it properly. I'm not sure if I see any other reasonable
solution besides these two options.

z