[PATCH v1] drm/i915/gvt: Don't replace a DMA mapping that is still in use

[Yuho Choi <oss.patchbox@gmail.com>]

intel_gvt_dma_map_guest_page() unmaps and frees the cache entry of a
gfn when it is requested with a different size, and maps the page again.
The entry is still referenced: shadow GTT entries and dma-bufs keep
using its DMA address after the page is unpinned and the DMA mapping is
torn down. When the new mapping gets the same DMA address, their later
intel_gvt_dma_unmap_guest_page() calls also drop the references of the
new entry.

Keep the existing entry instead. A 2M mapping covers a 4K request for
its first page, so take a reference on it. If the entry is smaller than
the request, fail and let the caller split the 2M entry into 4K pages,
as it already does when a 2M mapping cannot be set up.

Fixes: 7366aeb77cd8 ("drm/i915/gvt: fix incorrect cache entry for guest page mapping")
Cc: stable@vger.kernel.org
Signed-off-by: Yuho Choi <oss.patchbox@gmail.com>
---
Compile-tested only (x86_64 defconfig + DRM_I915_GVT_KVMGT, W=1, sparse).

 drivers/gpu/drm/i915/gvt/kvmgt.c | 20 ++++++++------------
 1 file changed, 8 insertions(+), 12 deletions(-)

diff --git a/drivers/gpu/drm/i915/gvt/kvmgt.c b/drivers/gpu/drm/i915/gvt/kvmgt.c
index ec62db5cc3675..9a3e305253469 100644
--- a/drivers/gpu/drm/i915/gvt/kvmgt.c
+++ b/drivers/gpu/drm/i915/gvt/kvmgt.c
@@ -1626,19 +1626,15 @@ int intel_gvt_dma_map_guest_page(struct intel_vgpu *vgpu, unsigned long gfn,
 		ret = __gvt_cache_add(vgpu, gfn, *dma_addr, size);
 		if (ret)
 			goto err_unmap;
-	} else if (entry->size != size) {
-		/* the same gfn with different size: unmap and re-map */
-		gvt_dma_unmap_page(vgpu, gfn, entry->dma_addr, entry->size);
-		__gvt_cache_remove_entry(vgpu, entry);
-
-		ret = gvt_dma_map_page(vgpu, gfn, dma_addr, size);
-		if (ret)
-			goto err_unlock;
-
-		ret = __gvt_cache_add(vgpu, gfn, *dma_addr, size);
-		if (ret)
-			goto err_unmap;
+	} else if (entry->size < size) {
+		/*
+		 * The smaller mapping may still be in use, so don't replace
+		 * it. Fail and let the caller map the range in smaller pages.
+		 */
+		ret = -EBUSY;
+		goto err_unlock;
 	} else {
+		/* A mapping of the same or a larger size covers the request */
 		kref_get(&entry->ref);
 		*dma_addr = entry->dma_addr;
 	}

base-commit: 7704c4c5bb127673b4f0ead839919db573559e38
-- 
2.43.0