netdev - Re: [PATCH net-next v3 7/7] bnxt_en: add support for device memory tcp

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Message-ID: <18255eaf-a2ce-4cd1-b47b-2482b6c42e08@davidwei.uk>
Date: Mon, 7 Oct 2024 19:57:00 -0700
From: David Wei <dw@...idwei.uk>
To: Taehee Yoo <ap420073@...il.com>, Mina Almasry <almasrymina@...gle.com>
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
 edumazet@...gle.com, netdev@...r.kernel.org, linux-doc@...r.kernel.org,
 donald.hunter@...il.com, corbet@....net, michael.chan@...adcom.com,
 kory.maincent@...tlin.com, andrew@...n.ch, maxime.chevallier@...tlin.com,
 danieller@...dia.com, hengqi@...ux.alibaba.com, ecree.xilinx@...il.com,
 przemyslaw.kitszel@...el.com, hkallweit1@...il.com, ahmed.zaki@...el.com,
 paul.greenwalt@...el.com, rrameshbabu@...dia.com, idosch@...dia.com,
 asml.silence@...il.com, kaiyuanz@...gle.com, willemb@...gle.com,
 aleksander.lobakin@...el.com, sridhar.samudrala@...el.com, bcreeley@....com,
 David Wei <dw@...idwei.uk>
Subject: Re: [PATCH net-next v3 7/7] bnxt_en: add support for device memory
 tcp

On 2024-10-04 03:34, Taehee Yoo wrote:
> On Fri, Oct 4, 2024 at 3:43 AM Mina Almasry <almasrymina@...gle.com> wrote:
>>> @@ -3608,9 +3629,11 @@ static void bnxt_free_rx_rings(struct bnxt *bp)
>>>
>>>  static int bnxt_alloc_rx_page_pool(struct bnxt *bp,
>>>                                    struct bnxt_rx_ring_info *rxr,
>>> +                                  int queue_idx,
>>>                                    int numa_node)
>>>  {
>>>         struct page_pool_params pp = { 0 };
>>> +       struct netdev_rx_queue *rxq;
>>>
>>>         pp.pool_size = bp->rx_agg_ring_size;
>>>         if (BNXT_RX_PAGE_MODE(bp))
>>> @@ -3621,8 +3644,15 @@ static int bnxt_alloc_rx_page_pool(struct bnxt *bp,
>>>         pp.dev = &bp->pdev->dev;
>>>         pp.dma_dir = bp->rx_dir;
>>>         pp.max_len = PAGE_SIZE;
>>> -       pp.flags = PP_FLAG_DMA_MAP | PP_FLAG_DMA_SYNC_DEV;
>>> +       pp.order = 0;
>>> +
>>> +       rxq = __netif_get_rx_queue(bp->dev, queue_idx);
>>> +       if (rxq->mp_params.mp_priv)
>>> +               pp.flags = PP_FLAG_DMA_MAP | PP_FLAG_ALLOW_UNREADABLE_NETMEM;
>>
>> This is not the intended use of PP_FLAG_ALLOW_UNREADABLE_NETMEM.
>>
>> The driver should set PP_FLAG_ALLOW_UNREADABLE_NETMEM when it's able
>> to handle unreadable netmem, it should not worry about whether
>> rxq->mp_params.mp_priv is set or not.
>>
>> You should set PP_FLAG_ALLOW_UNREADABLE_NETMEM when HDS is enabled.
>> Let core figure out if mp_params.mp_priv is enabled. All the driver
>> needs to report is whether it's configured to be able to handle
>> unreadable netmem (which practically means HDS is enabled).
> 
> The reason why the branch exists here is the PP_FLAG_ALLOW_UNREADABLE_NETMEM
> flag can't be used with PP_FLAG_DMA_SYNC_DEV.
> 
>  228         if (pool->slow.flags & PP_FLAG_DMA_SYNC_DEV) {
>  229                 /* In order to request DMA-sync-for-device the page
>  230                  * needs to be mapped
>  231                  */
>  232                 if (!(pool->slow.flags & PP_FLAG_DMA_MAP))
>  233                         return -EINVAL;
>  234
>  235                 if (!pool->p.max_len)
>  236                         return -EINVAL;
>  237
>  238                 pool->dma_sync = true;                //here
>  239
>  240                 /* pool->p.offset has to be set according to the address
>  241                  * offset used by the DMA engine to start copying rx data
>  242                  */
>  243         }
> 
> If PP_FLAG_DMA_SYNC_DEV is set, page->dma_sync is set to true.
> 
> 347 int mp_dmabuf_devmem_init(struct page_pool *pool)
> 348 {
> 349         struct net_devmem_dmabuf_binding *binding = pool->mp_priv;
> 350
> 351         if (!binding)
> 352                 return -EINVAL;
> 353
> 354         if (!pool->dma_map)
> 355                 return -EOPNOTSUPP;
> 356
> 357         if (pool->dma_sync)                      //here
> 358                 return -EOPNOTSUPP;
> 359
> 360         if (pool->p.order != 0)
> 361                 return -E2BIG;
> 362
> 363         net_devmem_dmabuf_binding_get(binding);
> 364         return 0;
> 365 }
> 
> In the mp_dmabuf_devmem_init(), it fails when pool->dma_sync is true.

This won't work for io_uring zero copy into user memory. We need all
PP_FLAG_DMA_MAP | PP_FLAG_DMA_SYNC_DEV | PP_FLAG_ALLOW_UNREADABLE_NETMEM
set.

I agree with Mina that the driver should not be poking at the mp_priv
fields. How about setting all the flags and then letting the mp->init()
figure it out? mp_dmabuf_devmem_init() is called within page_pool_init()
so as long as it resets dma_sync if set I don't see any issues.

> 
> tcp-data-split can be used for normal cases, not only devmem TCP case.
> If we enable tcp-data-split and disable devmem TCP, page_pool doesn't
> have PP_FLAG_DMA_SYNC_DEV.
> So I think mp_params.mp_priv is still useful.
> 
> Thanks a lot,
> Taehee Yoo
> 
>>
>>
>> --
>> Thanks,
>> Mina