FAILED: patch "[PATCH] drm/xe: Limit sg segment size to PAGE_SIZE on Xen PV" failed to apply to 7.3-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 7.3-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-7.3.y
git checkout FETCH_HEAD
git cherry-pick -x 77f704158f099b952681f207478a22d5b8218edb
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107050332-failed-7.3-77f704158f09@kernel.org' --subject-prefix 'PATCH 7.3.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 77f704158f099b952681f207478a22d5b8218edb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Szymon=20Aceda=C5=84ski?= <accek@invisiblethingslab.com>
Date: Wed, 16 Sep 2026 19:30:30 +0200
Subject: [PATCH] drm/xe: Limit sg segment size to PAGE_SIZE on Xen PV
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Fix display corruption on Xen PV dom0, where DMA buffers are not
guaranteed machine-contiguous, in which case bounce buffering kicks
in, breaking xe's memory coherency assumptions.

Apply the same workaround i915 carries in i915_sg_segment_size() since
commit 78a07fe777c4 ("drm/i915: stop abusing swiotlb_max_segment").

Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/8382
Link: https://lore.kernel.org/xen-devel/aYtznP_tT6xNPwf-@mail-itl/
Link: https://lore.kernel.org/all/20221020110308.1582518-1-hch@lst.de/ # i915 counterpart
Cc: Christoph Hellwig <hch@lst.de>
Cc: Robert Beckett <bob.beckett@collabora.com>
Cc: stable@vger.kernel.org # v6.8+
Signed-off-by: Szymon Acedański <accek@invisiblethingslab.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patch.msgid.link/20260916173030.3223833-1-accek@invisiblethingslab.com
---
 drivers/gpu/drm/xe/xe_bo.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/gpu/drm/xe/xe_bo.h b/drivers/gpu/drm/xe/xe_bo.h
index 290ca624e2a77..341fa93a71e45 100644
--- a/drivers/gpu/drm/xe/xe_bo.h
+++ b/drivers/gpu/drm/xe/xe_bo.h
@@ -9,6 +9,8 @@
 #include <drm/drm_prime.h>
 #include <drm/ttm/ttm_tt.h>
 
+#include <xen/xen.h>
+
 #include "xe_bo_types.h"
 #include "xe_ggtt.h"
 #include "xe_macros.h"
@@ -574,6 +576,23 @@ static inline unsigned int xe_sg_segment_size(struct device *dev)
 	struct scatterlist __maybe_unused sg;
 	size_t max = BIT_ULL(sizeof(sg.length) * 8) - 1;
 
+	/*
+	 * For Xen PV guests pages aren't contiguous in DMA (machine) address
+	 * space.  The DMA API takes care of that both in dma_alloc_* (by
+	 * calling into the hypervisor to make the pages contiguous) and in
+	 * dma_map_* (by bounce buffering).  But xe (like i915, see commit
+	 * 78a07fe777c4) ignores the coherency aspects of the DMA API and thus
+	 * can't cope with bounce buffering actually happening, so add a hack
+	 * here to force small allocations and mappings when running in PV
+	 * mode on Xen.
+	 *
+	 * Note this will still break if bounce buffering is required for other
+	 * reasons, like confidential computing hypervisors or PCIe root ports
+	 * with addressing limitations.
+	 */
+	if (xen_pv_domain())
+		return PAGE_SIZE;
+
 	max = min_t(size_t, max, dma_max_mapping_size(dev));
 
 	/*
-- 
2.53.0