Re: [PATCH 9/9] vfio/pci: Add mmap() attributes to DMABUF feature

[Leon Romanovsky]

On Fri, Apr 24, 2026 at 03:31:53PM -0300, Jason Gunthorpe wrote:
> On Thu, Apr 16, 2026 at 06:17:52AM -0700, Matt Evans wrote:
> > A new field is reserved in vfio_device_feature_dma_buf.flags to
> > request CPU-facing memory type attributes for mmap()s of the buffer.
> > Add a flag VFIO_DEVICE_FEATURE_DMA_BUF_ATTR_WC, which results in WC
> > PTEs for the DMABUF's BAR region.
> > 
> > Signed-off-by: Matt Evans <mattev@xxxxxxxx>
> > ---
> >  drivers/vfio/pci/vfio_pci_dmabuf.c | 15 +++++++++++++--
> >  drivers/vfio/pci/vfio_pci_priv.h   |  1 +
> >  include/uapi/linux/vfio.h          | 12 +++++++++---
> >  3 files changed, 23 insertions(+), 5 deletions(-)
> 
> Nice and simple
> 
> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> 
> > @@ -1549,8 +1551,12 @@ struct vfio_region_dma_range {
> >  struct vfio_device_feature_dma_buf {
> >  	__u32	region_index;
> >  	__u32	open_flags;
> > -	__u32   flags;
> > -	__u32   nr_ranges;
> > +	__u32	flags;
> > +	/* Flags sub-field reserved for attribute enum */
> > +#define VFIO_DEVICE_FEATURE_DMA_BUF_ATTR_MASK		(0xfU << 28)
> > +#define VFIO_DEVICE_FEATURE_DMA_BUF_ATTR_UC		(0 << 28)
> > +#define VFIO_DEVICE_FEATURE_DMA_BUF_ATTR_WC		(1 << 28)
> > +	__u32	nr_ranges;

Alex,

The TPH proposal extends the flags field in a similar way, but I suggested
a different approach to conserve bits. At the moment, we spend three bits
on a single feature, which feels wasteful.

What do you think?
https://lore.kernel.org/all/20260409120415.GF86584@unreal/

Thanks