drm/vmwgfx: Enable DMA mappings with SEV

Enable DMA mappings in vmwgfx after TTM has been fixed in commit
3bf3710e ("drm/ttm: Add a generic TTM memcpy move for page-based iomem")

This enables full guest-backed memory support and in particular allows
usage of screen targets as the presentation mechanism.
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Reported-by: Ye Li <ye.li@broadcom.com>
Tested-by: Ye Li <ye.li@broadcom.com>
Fixes: 3b0d6458 ("drm/vmwgfx: Refuse DMA operation when SEV encryption is active")
Cc: Broadcom internal kernel review list <bcm-kernel-feedback-list@broadcom.com>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v6.6+
Reviewed-by: Martin Krastev <martin.krastev@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240408022802.358641-1-zack.rusin@broadcom.com

drivers/gpu/drm/vmwgfx/vmwgfx_drv.c

@@ -666,11 +666,12 @@ static int vmw_dma_select_mode(struct vmw_private *dev_priv)
 		[vmw_dma_map_populate] = "Caching DMA mappings.",
 		[vmw_dma_map_bind] = "Giving up DMA mappings early."};
 
-	/* TTM currently doesn't fully support SEV encryption. */
-	if (cc_platform_has(CC_ATTR_MEM_ENCRYPT))
-		return -EINVAL;
-
-	if (vmw_force_coherent)
+	/*
+	 * When running with SEV we always want dma mappings, because
+	 * otherwise ttm tt pool pages will bounce through swiotlb running
+	 * out of available space.
+	 */
+	if (vmw_force_coherent || cc_platform_has(CC_ATTR_MEM_ENCRYPT))
 		dev_priv->map_mode = vmw_dma_alloc_coherent;
 	else if (vmw_restrict_iommu)
 		dev_priv->map_mode = vmw_dma_map_bind;