netdev - Re: [RFC PATCH net-next v6 09/15] memory-provider: dmabuf devmem memory provider

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Date: Tue, 5 Mar 2024 18:42:35 -0800
From: Mina Almasry <almasrymina@...gle.com>
To: David Wei <dw@...idwei.uk>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
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Subject: Re: [RFC PATCH net-next v6 09/15] memory-provider: dmabuf devmem
 memory provider

On Tue, Mar 5, 2024 at 6:28 PM David Wei <dw@...idwei.uk> wrote:
>
> On 2024-03-04 18:01, Mina Almasry wrote:
> > +     if (pool->p.queue)
> > +             binding = READ_ONCE(pool->p.queue->binding);
> > +
> > +     if (binding) {
> > +             pool->mp_ops = &dmabuf_devmem_ops;
> > +             pool->mp_priv = binding;
> > +     }
>
> This is specific to TCP devmem. For ZC Rx we will need something more
> generic to let us pass our own memory provider backend down to the page
> pool.
>
> What about storing ops and priv void ptr in struct netdev_rx_queue
> instead? Then we can both use it.

Yes, this is dmabuf specific, I was thinking you'd define your own
member of netdev_rx_queue, and then add something like this to
page_pool_init:

+       if (pool->p.queue)
+               io_uring_metadata = READ_ONCE(pool->p.queue->io_uring_metadata);
+
+       /* We don't support rx-queues that are configured for both
io_uring & dmabuf binding */
+       BUG_ON(io_uring_metadata && binding);
+
+       if (io_uring_metadata) {
+               pool->mp_ops = &io_uring_ops;
+               pool->mp_priv = io_uring_metadata;
+       }

I.e., we share the pool->mp_ops and the pool->mp_priv but we don't
really need to share the same netdev_rx_queue member. For me it's a
dma-buf specific data structure (netdev_dmabuf_binding) and for you
it's something else.

page_pool_init() probably needs to validate that the queue is
configured for dma-buf or io_uring but not both. If it's configured
for both then the user is doing something funky we shouldn't support.

Perhaps I can make the intention clearer by renaming 'binding' to
something more specific to dma-buf like queue->dmabuf_binding, to make
it clear that this is the dma-buf binding and not some other binding
like io_uring?

-- 
Thanks,
Mina