Re: [PATCH v2] drm/ttm: Make sure the mapped tt pages are decrypted when needed

[Zack Rusin]

On Mon, 2023-10-02 at 16:27 +0200, Thomas Hellström wrote:
> !! External Email
> 
> On Mon, 2023-10-02 at 10:16 +0200, Thomas Hellström wrote:
> > Hi, Zack
> > 
> > On 9/26/23 19:51, Zack Rusin wrote:
> > > From: Zack Rusin <zackr@xxxxxxxxxx>
> > > 
> > > Some drivers require the mapped tt pages to be decrypted. In an
> > > ideal
> > > world this would have been handled by the dma layer, but the TTM
> > > page
> > > fault handling would have to be rewritten to able to do that.
> > > 
> > > A side-effect of the TTM page fault handling is using a dma
> > > allocation
> > > per order (via ttm_pool_alloc_page) which makes it impossible to
> > > just
> > > trivially use dma_mmap_attrs. As a result ttm has to be very
> > > careful
> > > about trying to make its pgprot for the mapped tt pages match what
> > > the dma layer thinks it is. At the ttm layer it's possible to
> > > deduce the requirement to have tt pages decrypted by checking
> > > whether coherent dma allocations have been requested and the system
> > > is running with confidential computing technologies.
> > > 
> > > This approach isn't ideal but keeping TTM matching DMAs
> > > expectations
> > > for the page properties is in general fragile, unfortunately proper
> > > fix would require a rewrite of TTM's page fault handling.
> > > 
> > > Fixes vmwgfx with SEV enabled.
> > > 
> > > v2: Explicitly include cc_platform.h
> > > 
> > > Signed-off-by: Zack Rusin <zackr@xxxxxxxxxx>
> > > Fixes: 3bf3710e3718 ("drm/ttm: Add a generic TTM memcpy move for
> > > page-based iomem")
> > > Cc: Christian König <christian.koenig@xxxxxxx>
> > > Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> > > Cc: Huang Rui <ray.huang@xxxxxxx>
> > > Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > Cc: <stable@xxxxxxxxxxxxxxx> # v5.14+
> > > ---
> > >   drivers/gpu/drm/ttm/ttm_bo_util.c | 13 +++++++++++--
> > >   drivers/gpu/drm/ttm/ttm_tt.c      |  8 ++++++++
> > >   include/drm/ttm/ttm_tt.h          |  9 ++++++++-
> > >   3 files changed, 27 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c
> > > b/drivers/gpu/drm/ttm/ttm_bo_util.c
> > > index fd9fd3d15101..0b3f4267130c 100644
> > > --- a/drivers/gpu/drm/ttm/ttm_bo_util.c
> > > +++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
> > > @@ -294,7 +294,13 @@ pgprot_t ttm_io_prot(struct ttm_buffer_object
> > > *bo, struct ttm_resource *res,
> > >         enum ttm_caching caching;
> > > 
> > >         man = ttm_manager_type(bo->bdev, res->mem_type);
> > > -       caching = man->use_tt ? bo->ttm->caching : res-
> > > > bus.caching;
> > > +       if (man->use_tt) {
> > > +               caching = bo->ttm->caching;
> > > +               if (bo->ttm->page_flags & TTM_TT_FLAG_DECRYPTED)
> > > +                       tmp = pgprot_decrypted(tmp);
> > > +       } else  {
> > > +               caching = res->bus.caching;
> > > +       }
> > > 
> > >         return ttm_prot_from_caching(caching, tmp);
> > >   }
> > > @@ -337,6 +343,8 @@ static int ttm_bo_kmap_ttm(struct
> > > ttm_buffer_object *bo,
> > >                 .no_wait_gpu = false
> > >         };
> > >         struct ttm_tt *ttm = bo->ttm;
> > > +       struct ttm_resource_manager *man =
> > > +                       ttm_manager_type(bo->bdev, bo->resource-
> > > > mem_type);
> > >         pgprot_t prot;
> > >         int ret;
> > > 
> > > @@ -346,7 +354,8 @@ static int ttm_bo_kmap_ttm(struct
> > > ttm_buffer_object *bo,
> > >         if (ret)
> > >                 return ret;
> > > 
> > > -       if (num_pages == 1 && ttm->caching == ttm_cached) {
> > > +       if (num_pages == 1 && ttm->caching == ttm_cached &&
> > > +           !(man->use_tt && (ttm->page_flags &
> > > TTM_TT_FLAG_DECRYPTED))) {
> > >                 /*
> > >                  * We're mapping a single page, and the desired
> > >                  * page protection is consistent with the bo.
> > > diff --git a/drivers/gpu/drm/ttm/ttm_tt.c
> > > b/drivers/gpu/drm/ttm/ttm_tt.c
> > > index e0a77671edd6..e4966e2c988d 100644
> > > --- a/drivers/gpu/drm/ttm/ttm_tt.c
> > > +++ b/drivers/gpu/drm/ttm/ttm_tt.c
> > > @@ -31,6 +31,7 @@
> > > 
> > >   #define pr_fmt(fmt) "[TTM] " fmt
> > > 
> > > +#include <linux/cc_platform.h>
> > >   #include <linux/sched.h>
> > >   #include <linux/shmem_fs.h>
> > >   #include <linux/file.h>
> > > @@ -81,6 +82,13 @@ int ttm_tt_create(struct ttm_buffer_object *bo,
> > > bool zero_alloc)
> > >                 pr_err("Illegal buffer object type\n");
> > >                 return -EINVAL;
> > >         }
> > > +       /*
> > > +        * When using dma_alloc_coherent with memory encryption the
> > > +        * mapped TT pages need to be decrypted or otherwise the
> > > drivers
> > > +        * will end up sending encrypted mem to the gpu.
> > > +        */
> > > +       if (bdev->pool.use_dma_alloc &&
> > > cc_platform_has(CC_ATTR_MEM_ENCRYPT))
> > 
> > You need to use CC_ATTR_GUEST_MEM_ENCRYPT here rather than
> > CC_ATTR_MEM_ENCRYPT to avoid touching and breaking the SME case and
> > only
> > fix the SEV / SEV-ES case. I'd also hold off the stable inclusion
> > until
> > it's completely verified that this doesn't break anything because if
> > it
> > does, I suspect all hell will break loose.
> > 
> > With that said, for the functionality
> > 
> > Reviewed-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> > 
> > But I think this needs a wider Ack at the ttm / drm level for the
> > approach taken.
> > 
> > /Thomas.
> 
> FWIW, I think that if TTM_TT_FLAG_DECRYPTED is set, it should be
> possible to add a debug WARN_ON_ONCE() if the first PTE of the dma
> page's kernel virtual address does not use a decrypted pgprot_t. One
> way of accessing the PTEs in a platform-generic fashion is
> apply_to_page_range().

Good point. 

Another, trivial solution to that problem of possible regression would simply be
introducing:

#define TTM_DEVICE_USE_DMA_ALLOC          BIT(0)
#define TTM_DEVICE_USE_GFP_DMA32          BIT(1)
#define TTM_DEVICE_USE_DECRYPTED_SYS_MEM  BIT(2)

and changing ttm_device_init from:

int ttm_device_init(struct ttm_device *bdev, const struct ttm_device_funcs *funcs,
		    struct device *dev, struct address_space *mapping,
		    struct drm_vma_offset_manager *vma_manager,
		    bool use_dma_alloc, bool use_dma32);
to:
int ttm_device_init(struct ttm_device *bdev, const struct ttm_device_funcs *funcs,
		    struct device *dev, struct address_space *mapping,
		    struct drm_vma_offset_manager *vma_manager,
		    u32 use_flags);

The driver should have a lot clearer picture of whether
TTM_DEVICE_USE_DECRYPTED_SYS_MEM should be used. That change requires porting the
drivers to the new ttm_device_init (which is trivial) but guarantees no regressions
simply by virture of having vmwgfx use TTM_DEVICE_USE_DECRYPTED_SYS_MEM only (at
least initially, I imagine at least qxl would need it as well).

Christian, any thoughts?

z