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

[Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>]

On Mon, 2026-09-21 at 15:57 +0200, Thomas Hellström wrote:
> 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>

Pushed to drm-xe-next. 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 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