Re: [PATCH v4 3/3] hw/vfio/region: Create dmabuf for PCI BAR per region

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Aug 05, 2026 at 08:33:06AM +0000, Shameer Kolothum Thodi wrote:
> > > +    dma_buf = (void *)feature->data;
> > > +    *dma_buf = (struct vfio_device_feature_dma_buf) {
> > > +        .region_index = region->nr,
> > > +        .open_flags = O_RDWR,
> > > +        .nr_ranges = region->nr_mmaps,
> > > +    };
> > > +
> > > +    for (i = 0; i < region->nr_mmaps; i++) {
> > > +        dma_buf->dma_ranges[i].offset = region->mmaps[i].offset;
> > > +        dma_buf->dma_ranges[i].length = region->mmaps[i].size;
> > > +    }
> > > +
> > 
> > Shall we check if @offset and @size is aligned to PAGE_SIZE? If they're not,
> > I guess we need to skip populating DMA buffer instead of preventing the
> > device from being passed through to the guest.
> 
> Hmm..does it prevent passthrough in this case? The intention was not to do that.
> vfio_region_mmap() returns 0 irrespective of dma-buf outcome.

I think you should always be able to create a dmabuf, even with
irregular offsets and sizes.

The resulting dmabuf may not be mmap'able though.

Jason