Re: [PATCH v5 9/9] vfio/pci: Add dma-buf export support for MMIO regions

Leon Romanovsky
2025-10-17T05:40:11Z

On Thu, Oct 16, 2025 at 08:53:32PM -0300, Jason Gunthorpe wrote:
> On Mon, Oct 13, 2025 at 06:26:11PM +0300, Leon Romanovsky wrote:
> > +
> > +static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf,
> > + struct dma_buf_attachment *attachment)
> > +{
> > + struct vfio_pci_dma_buf *priv = dmabuf->priv;
> > +
> > + if (!attachment->peer2peer)
> > + return -EOPNOTSUPP;
> > +
> > + if (priv->revoked)
> > + return -ENODEV;
> > +
> > + switch (pci_p2pdma_map_type(priv->provider, attachment->dev)) {
> > + case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
> > + break;
> > + case PCI_P2PDMA_MAP_BUS_ADDR:
> > + /*
> > + * There is no need in IOVA at all for this flow.
> > + * We rely on attachment->priv == NULL as a marker
> > + * for this mode.
> > + */
> > + return 0;
> > + default:
> > + return -EINVAL;
> > + }
> > +
> > + attachment->priv = kzalloc(sizeof(struct dma_iova_state), GFP_KERNEL);
> > + if (!attachment->priv)
> > + return -ENOMEM;
> > +
> > + dma_iova_try_alloc(attachment->dev, attachment->priv, 0, priv->size);
>
> The lifetime of this isn't good..
>
> > + return 0;
> > +}
> > +
> > +static void vfio_pci_dma_buf_detach(struct dma_buf *dmabuf,
> > + struct dma_buf_attachment *attachment)
> > +{
> > + kfree(attachment->priv);
> > +}
>
> If the caller fails to call map then it leaks the iova.

I'm relying on dmabuf code and documentation:

926 /**
927 * dma_buf_dynamic_attach - Add the device to dma_buf's attachments list
...
932 *
933 * Returns struct dma_buf_attachment pointer for this attachment. Attachments
934 * must be cleaned up by calling dma_buf_detach().

Successful call to vfio_pci_dma_buf_attach() MUST be accompanied by call
to vfio_pci_dma_buf_detach(), so as far as dmabuf implementation follows
it, there is no leak.

>
> > +static struct sg_table *
> > +vfio_pci_dma_buf_map(struct dma_buf_attachment *attachment,
> > + enum dma_data_direction dir)
> > +{
> [..]
>
>
> > +err_unmap_dma:
> > + if (!i || !state)
> > + ; /* Do nothing */
> > + else if (dma_use_iova(state))
> > + dma_iova_destroy(attachment->dev, state, mapped_len, dir,
> > + attrs);
>
> If we hit this error path then it is freed..
>
> > +static void vfio_pci_dma_buf_unmap(struct dma_buf_attachment *attachment,
> > + struct sg_table *sgt,
> > + enum dma_data_direction dir)
> > +{
> > + struct vfio_pci_dma_buf *priv = attachment->dmabuf->priv;
> > + struct dma_iova_state *state = attachment->priv;
> > + unsigned long attrs = DMA_ATTR_MMIO;
> > + struct scatterlist *sgl;
> > + int i;
> > +
> > + if (!state)
> > + ; /* Do nothing */
> > + else if (dma_use_iova(state))
> > + dma_iova_destroy(attachment->dev, state, priv->size, dir,
> > + attrs);
>
> It is freed here too, but we can call map multiple times. Every time a
> move_notify happens can trigger another call to map.
>
> I think just call unlink in those two and put dma_iova_free in detach

Yes, it can work.

Thanks

>
> Jason