On Mon, 11 May 2026 16:30:39 +0100 Matt Evans <[email protected]> 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? 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. I think the question of how we actually expand an arbitrary grab bag of "ATTRS" is the central question in whether we should implement the interface. If we follow the direction I suggested for TPH, maybe this is just a VFIO_DEVICE_FEATURE_DMA_BUF_WC, where it supports only PROBE and SET, with SET taking only the dma-buf fd to implement the one-way promotion from UC -> WC. If we support a generic SET ATTRS feature, we really need to map out how flag bits are indicated as supported and how a user untangles failures from trying to set various attributes. If we end up with a feature indicating each ATTR is available, we might as well have just implemented a feature for each attribute. Thanks, Alex
