On Tue, Mar 5, 2024 at 1:05 AM Arnd Bergmann arnd@arndb.de wrote:
On Tue, Mar 5, 2024, at 03:01, Mina Almasry wrote:
+int netdev_bind_dmabuf(struct net_device *dev, unsigned int dmabuf_fd,
struct netdev_dmabuf_binding **out)+{
struct netdev_dmabuf_binding *binding;static u32 id_alloc_next;struct scatterlist *sg;struct dma_buf *dmabuf;unsigned int sg_idx, i;unsigned long virtual;int err;if (!capable(CAP_NET_ADMIN))return -EPERM;dmabuf = dma_buf_get(dmabuf_fd);if (IS_ERR_OR_NULL(dmabuf))return -EBADFD;You should never need to use IS_ERR_OR_NULL() for a properly defined kernel interface. This one should always return an error or a valid pointer, so don't check for NULL.
Thanks for clarifying. I will convert to IS_ERR().
binding->attachment = dma_buf_attach(binding->dmabuf, dev->dev.parent);if (IS_ERR(binding->attachment)) {err = PTR_ERR(binding->attachment);goto err_free_id;}binding->sgt =dma_buf_map_attachment(binding->attachment, DMA_BIDIRECTIONAL);if (IS_ERR(binding->sgt)) {err = PTR_ERR(binding->sgt);goto err_detach;}Should there be a check to verify that this buffer is suitable for network data?
In general, dmabuf allows buffers that are uncached or reside in MMIO space of another device, but I think this would break when you get an skb with those buffers and try to parse the data inside of the kernel on architectures where MMIO space is not a normal pointer or unaligned access is disallowed on uncached data.
Arnd
A key goal of this patch series is that the kernel does not try to parse the skb frags that reside in the dma-buf for that precise reason. This is achieved using patch "net: add support for skbs with unreadable frags" which disables the kernel touching the payload in these skbs, and "tcp: RX path for devmem TCP" which implements a uapi where the kernel hands the data in the dmabuf to the userspace via a cmsg that gives the user a pointer to the data in the dmabuf (offset + size).
So really AFACT the only restriction here is that the NIC should be able to DMA into the dmabuf that we're attaching, and dma_buf_attach() fails in this scenario so we're covered there.