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:

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On Sun, 26 Apr 2026 13:52:15 +0300 Leon Romanovsky leon@kernel.org wrote:

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On Fri, Apr 24, 2026 at 03:31:53PM -0300, Jason Gunthorpe wrote:

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On Thu, Apr 16, 2026 at 06:17:52AM -0700, Matt Evans wrote:

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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@meta.com

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@nvidia.com

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@@ -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/20260423132016.4a25e074@shazbot.org/

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