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

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