linux-kernel - [PATCH] drm/virtio: Support partial maps of GEM objects

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Message-Id: <20250109-virtgpu-gem-partial-map-v1-1-a914b48776bd@gmail.com>
Date: Thu, 09 Jan 2025 22:45:46 +0100
From: Sasha Finkelstein via B4 Relay <devnull+fnkl.kernel.gmail.com@...nel.org>
To: David Airlie <airlied@...hat.com>, Gerd Hoffmann <kraxel@...hat.com>, 
 Gurchetan Singh <gurchetansingh@...omium.org>, 
 Chia-I Wu <olvaffe@...il.com>, 
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
 Simona Vetter <simona@...ll.ch>
Cc: dri-devel@...ts.freedesktop.org, virtualization@...ts.linux.dev, 
 linux-kernel@...r.kernel.org, asahi@...ts.linux.dev, 
 Sasha Finkelstein <fnkl.kernel@...il.com>
Subject: [PATCH] drm/virtio: Support partial maps of GEM objects

From: Sasha Finkelstein <fnkl.kernel@...il.com>

Those are useful to implement coherent cross-vm mmap.

Signed-off-by: Sasha Finkelstein <fnkl.kernel@...il.com>
---
 drivers/gpu/drm/virtio/virtgpu_vram.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_vram.c b/drivers/gpu/drm/virtio/virtgpu_vram.c
index 25df81c027837c248a746e41856b5aa7e216b8d5..64e2c6dbdd678ac4c0da89fdd4c9dbf937c2c335 100644
--- a/drivers/gpu/drm/virtio/virtgpu_vram.c
+++ b/drivers/gpu/drm/virtio/virtgpu_vram.c
@@ -56,12 +56,11 @@ static int virtio_gpu_vram_mmap(struct drm_gem_object *obj,
 	else if (vram->map_info == VIRTIO_GPU_MAP_CACHE_UNCACHED)
 		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
 
-	/* Partial mappings of GEM buffers don't happen much in practice. */
-	if (vm_size != vram->vram_node.size)
+	if (vm_size > vram->vram_node.size)
 		return -EINVAL;
 
 	ret = io_remap_pfn_range(vma, vma->vm_start,
-				 vram->vram_node.start >> PAGE_SHIFT,
+				 (vram->vram_node.start >> PAGE_SHIFT) + vma->vm_pgoff,
 				 vm_size, vma->vm_page_prot);
 	return ret;
 }

---
base-commit: 643e2e259c2b25a2af0ae4c23c6e16586d9fd19c
change-id: 20250109-virtgpu-gem-partial-map-335ec40656d1