On Mon, May 11, 2026 at 04:30:39PM +0100, Matt Evans wrote: > Hi Alex, Leon, > > On 27/04/2026 15:36, Alex Williamson wrote: > > > > On Sun, 26 Apr 2026 13:52:15 +0300 > > Leon Romanovsky <[email protected]> wrote: > > > > > 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 <[email protected]> > > > > > --- > > > > > 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 <[email protected]> > > > > > @@ -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/ > > > > I already proposed a very different interface for TPH that decouples > > the dma-buf creation from setting the TPH values: > > > > https://lore.kernel.org/all/[email protected]/ > > > > This is overall less intrusive than the TPH change proposed, but it > > could still make sense to align this as an operation on the dma-buf, > > that can be probed as a separate feature. Thanks, > > I'll add a VFIO_DEVICE_FEATURE_DMA_BUF_ATTRS in a v2 instead to get in line > with the TPH work, no worries. > > For the benefit of future hackers, how would you describe the criteria for > adding flags to this existing field?
One bit per-feature. > What hypothetical feature characteristics would be appropriate? (Maybe it's > that these attrs & TPH > add scalar fields in several bits rather than a simple boolean.) Two of us > have independently added something that's turned out to be inapproriate so > some guidance would be good. Both of you intertwined the signaling bit with the actual data, and that is what led me to prefer a different approach. Thanks > > Thanks! > > > Matt
