drm/ttm: Make sure the mapped tt pages are decrypted when needed

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
v3: Use CC_ATTR_GUEST_MEM_ENCRYPT instead of CC_ATTR_MEM_ENCRYPT to
limit the scope to guests and log when memory decryption is enabled.

Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Fixes: 3bf3710e3718 ("drm/ttm: Add a generic TTM memcpy move for page-based iomem")
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Christian König <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-kernel@vger.kernel.org
Cc: <stable@vger.kernel.org> # v5.14+
Link: https://patchwork.freedesktop.org/patch/msgid/20230926040359.3040017-1-zack@kde.org

2 files changed, 23 insertions, 2 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 d978dc539a9b..578a7c37f00b 100644
--- a/drivers/gpu/drm/ttm/ttm_tt.c
+++ b/drivers/gpu/drm/ttm/ttm_tt.c
@@ -31,11 +31,13 @@
 
 #define pr_fmt(fmt) "[TTM] " fmt
 
+#include <linux/cc_platform.h>
 #include <linux/sched.h>
 #include <linux/shmem_fs.h>
 #include <linux/file.h>
 #include <linux/module.h>
 #include <drm/drm_cache.h>
+#include <drm/drm_device.h>
 #include <drm/drm_util.h>
 #include <drm/ttm/ttm_bo.h>
 #include <drm/ttm/ttm_tt.h>
@@ -61,6 +63,7 @@ static atomic_long_t ttm_dma32_pages_allocated;
 int ttm_tt_create(struct ttm_buffer_object *bo, bool zero_alloc)
 {
 	struct ttm_device *bdev = bo->bdev;
+	struct drm_device *ddev = bo->base.dev;
 	uint32_t page_flags = 0;
 
 	dma_resv_assert_held(bo->base.resv);
@@ -82,6 +85,15 @@ 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_GUEST_MEM_ENCRYPT)) {
+		page_flags |= TTM_TT_FLAG_DECRYPTED;
+		drm_info(ddev, "TT memory decryption enabled.");
+	}
 
 	bo->ttm = bdev->funcs->ttm_tt_create(bo, page_flags);
 	if (unlikely(bo->ttm == NULL))