Re: [PATCH v2] drm/xe: Limit sg segment size to PAGE_SIZE on Xen PV

[Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>]

On Wed, 2026-09-16 at 19:30 +0200, Szymon Acedański wrote:
> 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@xxxxxxxxxxxxxxxxxxxxxx>
> 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@xxxxxx/ #
> i915 counterpart
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Robert Beckett <bob.beckett@xxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # v6.8+
> Signed-off-by: Szymon Acedański <accek@xxxxxxxxxxxxxxxxxxxxxx>
> ---
> v2:
>  - Imperative language in the commit message (Thomas Hellström)
>  - CC the authors of the original i915 workaround (Thomas Hellström)

Reviewed-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>


> 
>  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 290ca62..341fa93 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));
>  
>       /*
> 
> base-commit: baafc300cd079a5210c1e70e5ea3d93518e40b38