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? 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.
Thanks!
Matt
