Re: [PATCH RFC 12/26] vfio/pci: Change the DMA-buf exporter to use mapping_type

Baolu Lu
2026-04-13T08:31:52Z

On 2/18/26 08:11, Jason Gunthorpe wrote:
> Simple conversion to add a match_mapping() callback that offers an
> exporter SGT mapping type. Later patches will add a physical address
> exporter so go straight to adding the match_mapping() function.
>
> The check for attachment->peer2peer is replaced with setting
> exporter_requires_p2p=true. VFIO always uses MMIO memory.
>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
> drivers/vfio/pci/vfio_pci_dmabuf.c | 31 +++++++++++++++++++++++++-----
> 1 file changed, 26 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
> index d4d0f7d08c53e2..c7addef5794abf 100644
> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> @@ -25,9 +25,6 @@ static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf,
> {
> struct vfio_pci_dma_buf *priv = dmabuf->priv;
>
> - if (!attachment->peer2peer)
> - return -EOPNOTSUPP;
> -
> if (priv->revoked)
> return -ENODEV;
>
> @@ -75,11 +72,35 @@ static void vfio_pci_dma_buf_release(struct dma_buf *dmabuf)
> kfree(priv);
> }
>
> -static const struct dma_buf_ops vfio_pci_dmabuf_ops = {
> - .attach = vfio_pci_dma_buf_attach,
> +static const struct dma_buf_mapping_sgt_exp_ops vfio_pci_dma_buf_sgt_ops = {
> .map_dma_buf = vfio_pci_dma_buf_map,
> .unmap_dma_buf = vfio_pci_dma_buf_unmap,
> +};
> +
> +static int vfio_pci_dma_buf_match_mapping(struct dma_buf_match_args *args)
> +{
> + struct vfio_pci_dma_buf *priv = args->dmabuf->priv;
> + struct dma_buf_mapping_match sgt_match[1];
> +
> + dma_resv_assert_held(priv->dmabuf->resv);

My understanding of this lock assertion is that priv and the underlying
priv->vdev are accessed within this function. Therefore, the lock is
necessary to protect them. Do I understand it right?

However, callers - for example, dma_buf_mapping_attach() - do not
acquire dma_resv_lock() before calling this function. So kernel traces
will always be triggered.

> +
> + /*
> + * Once we pass vfio_pci_dma_buf_cleanup() the dmabuf will never be
> + * usable again.
> + */
> + if (!priv->vdev)
> + return -ENODEV;
> +
> + sgt_match[0] = DMA_BUF_EMAPPING_SGT_P2P(&vfio_pci_dma_buf_sgt_ops,
> + priv->vdev->pdev);
> +
> + return dma_buf_match_mapping(args, sgt_match, ARRAY_SIZE(sgt_match));
> +}
> +
> +static const struct dma_buf_ops vfio_pci_dmabuf_ops = {
> + .attach = vfio_pci_dma_buf_attach,
> .release = vfio_pci_dma_buf_release,
> + .match_mapping = vfio_pci_dma_buf_match_mapping,
> };
>
> /*

Thanks,
baolu