linux-kernel - Re: [PATCH net-next v18 07/14] memory-provider: dmabuf devmem memory provider

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Message-ID: <CAHS8izPyGwe_i4eNemW+A+MgMVHqJ0fdp=+-ju2ynqgc0mb_Ow@mail.gmail.com>
Date: Tue, 13 Aug 2024 04:39:47 -0400
From: Mina Almasry <almasrymina@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Pavel Begunkov <asml.silence@...il.com>, netdev@...r.kernel.org, 
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Subject: Re: [PATCH net-next v18 07/14] memory-provider: dmabuf devmem memory provider

On Mon, Aug 12, 2024 at 8:15 PM Jakub Kicinski <kuba@...nel.org> wrote:
> BTW, Mina, the core should probably also check that XDP isn't installed
> before / while the netmem is bound to a queue.

Sorry if noob question, but what is the proper check for this? I tried
adding this to net_devmem_bind_dmabuf_to_queue():

if (xdp_rxq_info_is_reg(&rxq->xdp_rxq))
                 return -EEXIST;

But quickly found out that in  netif_alloc_rx_queues() we initialize
all the rxq->xdp_rxq to state REGISTERED regardless whether xdp is
installed or not, so this check actually fails.

Worthy of note is that GVE holds an instance of xdp_rxq_info in
gve_rx_ring, and seems to use that for its xdp information, not the
one that hangs off of netdev_rx_queue in core.

Additionally, my understanding of XDP is limited, but why do we want
to disable it? My understanding is that XDP is a kernel bypass that
hands the data directly to userspace. In theory at least there should
be no issue binding dmabuf to a queue, then getting the data in the
queue via an XDP program instead of via TCP sockets or io uring. Is
there some fundamental reason why dmabuf and XDP are incompatible?

-- 
Thanks,
Mina