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

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

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
> <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: stable@vger.kernel.org # v6.8+
> Signed-off-by: Szymon Acedański <accek@invisiblethingslab.com>

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