linux-kernel - Re: [RFC PATCH net-next v6 05/15] netdev: support binding dma-buf to netdevice

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Date: Tue, 5 Mar 2024 12:00:08 -0800
From: Mina Almasry <almasrymina@...gle.com>
To: Arnd Bergmann <arnd@...db.de>
Cc: Netdev <netdev@...r.kernel.org>, linux-kernel@...r.kernel.org, 
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	Willem de Bruijn <willemb@...gle.com>, Kaiyuan Zhang <kaiyuanz@...gle.com>
Subject: Re: [RFC PATCH net-next v6 05/15] netdev: support binding dma-buf to netdevice

On Tue, Mar 5, 2024 at 1:05 AM Arnd Bergmann <arnd@...db.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.

-- 
Thanks,
Mina