Hi!
On Thu, 2026-08-20 at 12:12 +0200, Szymon Acedański wrote:
> Fixes display corruption on Xen PV dom0, where DMA buffers are not
Please use imperative language in commit messages: "Fix display
corruption..."
> 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
> <[email protected]>
> 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/[email protected]/ #
> i915 counterpart
> Cc: [email protected] # v6.8+
> Signed-off-by: Szymon Acedański <[email protected]>
Please also CC the Author of the original i915 patch in case there
are any updates to the validity of this hack. While there is a
precedent in i915 authored by Christoph, the patch really relies
on completely undocumented behaviour....
Otherwise LGTM, Once you have an update I'll forward it to Xe CI.
Thanks,
Thomas
> ---
> 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 e8081af..152bfcf 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"
> @@ -575,6 +577,23 @@ 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));
>
> /*
>
> base-commit: b4f95affc66ef76342c1f6bf3849f6c8ade6b9d6
