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

[Alex Williamson]

On Mon, 11 May 2026 16:30:39 +0100
Matt Evans <mattev@xxxxxxxx> wrote:

> Hi Alex, Leon,
> 
> On 27/04/2026 15:36, Alex Williamson wrote:
> > 
> > On Sun, 26 Apr 2026 13:52:15 +0300
> > Leon Romanovsky <leon@xxxxxxxxxx> 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 <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/  
> > 
> > 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@xxxxxxxxxxx/
> > 
> > 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