Subject: Re: [xdp-hints] Re: [PATCH bpf-next v7 15/17] net/mlx5e: Introduce wrapper for xdp_buff

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Fri, 13 Jan 2023 22:31:59 +0100

Tariq Toukan <[email-masked]> writes:

> On 12/01/2023 23:55, Toke Høiland-Jørgensen wrote:
>> Toke Høiland-Jørgensen <[email-masked]> writes:
>> 
>>> Stanislav Fomichev <[email-masked]> writes:
>>>
>>>> On Thu, Jan 12, 2023 at 12:07 AM Tariq Toukan <[email-masked]> wrote:
>>>>>
>>>>>
>>>>>
>>>>> On 12/01/2023 2:32, Stanislav Fomichev wrote:
>>>>>> From: Toke Høiland-Jørgensen <[email-masked]>
>>>>>>
>>>>>> Preparation for implementing HW metadata kfuncs. No functional change.
>>>>>>
>>>>>> Cc: Tariq Toukan <[email-masked]>
>>>>>> Cc: Saeed Mahameed <[email-masked]>
>>>>>> Cc: John Fastabend <[email-masked]>
>>>>>> Cc: David Ahern <[email-masked]>
>>>>>> Cc: Martin KaFai Lau <[email-masked]>
>>>>>> Cc: Jakub Kicinski <[email-masked]>
>>>>>> Cc: Willem de Bruijn <[email-masked]>
>>>>>> Cc: Jesper Dangaard Brouer <[email-masked]>
>>>>>> Cc: Anatoly Burakov <[email-masked]>
>>>>>> Cc: Alexander Lobakin <[email-masked]>
>>>>>> Cc: Magnus Karlsson <[email-masked]>
>>>>>> Cc: Maryam Tahhan <[email-masked]>
>>>>>> Cc: [email-masked]
>>>>>> Cc: [email-masked]
>>>>>> Signed-off-by: Toke Høiland-Jørgensen <[email-masked]>
>>>>>> Signed-off-by: Stanislav Fomichev <[email-masked]>
>>>>>> ---
>>>>>>    drivers/net/ethernet/mellanox/mlx5/core/en.h  |  1 +
>>>>>>    .../net/ethernet/mellanox/mlx5/core/en/xdp.c  |  3 +-
>>>>>>    .../net/ethernet/mellanox/mlx5/core/en/xdp.h  |  6 +-
>>>>>>    .../ethernet/mellanox/mlx5/core/en/xsk/rx.c   | 25 ++++----
>>>>>>    .../net/ethernet/mellanox/mlx5/core/en_rx.c   | 58 +++++++++----------
>>>>>>    5 files changed, 50 insertions(+), 43 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en.h b/drivers/net/ethernet/mellanox/mlx5/core/en.h
>>>>>> index 2d77fb8a8a01..af663978d1b4 100644
>>>>>> --- a/drivers/net/ethernet/mellanox/mlx5/core/en.h
>>>>>> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en.h
>>>>>> @@ -469,6 +469,7 @@ struct mlx5e_txqsq {
>>>>>>    union mlx5e_alloc_unit {
>>>>>>        struct page *page;
>>>>>>        struct xdp_buff *xsk;
>>>>>> +     struct mlx5e_xdp_buff *mxbuf;
>>>>>
>>>>> In XSK files below you mix usage of both alloc_units[page_idx].mxbuf and
>>>>> alloc_units[page_idx].xsk, while both fields share the memory of a union.
>>>>>
>>>>> As struct mlx5e_xdp_buff wraps struct xdp_buff, I think that you just
>>>>> need to change the existing xsk field type from struct xdp_buff *xsk
>>>>> into struct mlx5e_xdp_buff *xsk and align the usage.
>>>>
>>>> Hmmm, good point. I'm actually not sure how it works currently.
>>>> mlx5e_alloc_unit.mxbuf doesn't seem to be initialized anywhere? Toke,
>>>> am I missing something?
>>>
>>> It's initialised piecemeal in different places; but yeah, we're mixing
>>> things a bit...
>>>
>>>> I'm thinking about something like this:
>>>>
>>>> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en.h
>>>> b/drivers/net/ethernet/mellanox/mlx5/core/en.h
>>>> index af663978d1b4..2d77fb8a8a01 100644
>>>> --- a/drivers/net/ethernet/mellanox/mlx5/core/en.h
>>>> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en.h
>>>> @@ -469,7 +469,6 @@ struct mlx5e_txqsq {
>>>>   union mlx5e_alloc_unit {
>>>>          struct page *page;
>>>>          struct xdp_buff *xsk;
>>>> -       struct mlx5e_xdp_buff *mxbuf;
>>>>   };
>>>
>>> Hmm, for consistency with the non-XSK path we should rather go the other
>>> direction and lose the xsk member, moving everything to mxbuf? Let me
>>> give that a shot...
>> 
>> Something like the below?
>> 
>> -Toke
>> 
>> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en.h b/drivers/net/ethernet/mellanox/mlx5/core/en.h
>> index 6de02d8aeab8..cb9cdb6421c5 100644
>> --- a/drivers/net/ethernet/mellanox/mlx5/core/en.h
>> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en.h
>> @@ -468,7 +468,6 @@ struct mlx5e_txqsq {
>>   
>>   union mlx5e_alloc_unit {
>>   	struct page *page;
>> -	struct xdp_buff *xsk;
>>   	struct mlx5e_xdp_buff *mxbuf;
>>   };
>>   
>> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.h b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.h
>> index cb568c62aba0..95694a25ec31 100644
>> --- a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.h
>> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.h
>> @@ -33,6 +33,7 @@
>>   #define __MLX5_EN_XDP_H__
>>   
>>   #include <linux/indirect_call_wrapper.h>
>> +#include <net/xdp_sock_drv.h>
>>   
>>   #include "en.h"
>>   #include "en/txrx.h"
>> @@ -112,6 +113,21 @@ static inline void mlx5e_xmit_xdp_doorbell(struct mlx5e_xdpsq *sq)
>>   	}
>>   }
>>   
>> +static inline struct mlx5e_xdp_buff *mlx5e_xsk_buff_alloc(struct xsk_buff_pool *pool)
>> +{
>> +	return (struct mlx5e_xdp_buff *)xsk_buff_alloc(pool);
>
> What about the space needed for the rq / cqe fields? xsk_buff_alloc 
> won't allocate it.

It will! See patch 14 in the series that adds a 'cb' field to
xdp_buff_xsk, meaning there's actually space after the xdp_buff struct
being allocated by the xsk_buff_alloc API. The XSK_CHECK_PRIV_TYPE macro
call is there to ensure the cb field is big enough for the struct we're
casting to in the driver.

>> +}
>> +
>> +static inline void mlx5e_xsk_buff_free(struct mlx5e_xdp_buff *mxbuf)
>> +{
>> +	xsk_buff_free(&mxbuf->xdp);
>> +}
>> +
>> +static inline dma_addr_t mlx5e_xsk_buff_xdp_get_frame_dma(struct mlx5e_xdp_buff *mxbuf)
>> +{
>> +	return xsk_buff_xdp_get_frame_dma(&mxbuf->xdp);
>> +}
>> +
>>   /* Enable inline WQEs to shift some load from a congested HCA (HW) to
>>    * a less congested cpu (SW).
>>    */
>> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/rx.c
>> index 8bf3029abd3c..1f166dbb7f22 100644
>> --- a/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/rx.c
>> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/rx.c
>> @@ -3,7 +3,6 @@
>>   
>>   #include "rx.h"
>>   #include "en/xdp.h"
>> -#include <net/xdp_sock_drv.h>
>>   #include <linux/filter.h>
>>   
>>   /* RX data path */
>> @@ -21,7 +20,7 @@ int mlx5e_xsk_alloc_rx_mpwqe(struct mlx5e_rq *rq, u16 ix)
>>   	if (unlikely(!xsk_buff_can_alloc(rq->xsk_pool, rq->mpwqe.pages_per_wqe)))
>>   		goto err;
>>   
>> -	BUILD_BUG_ON(sizeof(wi->alloc_units[0]) != sizeof(wi->alloc_units[0].xsk));
>> +	BUILD_BUG_ON(sizeof(wi->alloc_units[0]) != sizeof(wi->alloc_units[0].mxbuf));
>>   	XSK_CHECK_PRIV_TYPE(struct mlx5e_xdp_buff);
>>   	batch = xsk_buff_alloc_batch(rq->xsk_pool, (struct xdp_buff **)wi->alloc_units,
>>   				     rq->mpwqe.pages_per_wqe);
>
> This batching API gets broken as well...
> xsk_buff_alloc_batch fills an array of struct xdp_buff pointers, it 
> cannot correctly act on the array of struct mlx5e_xdp_buff, as it 
> contains additional fields.

See above for why this does, in fact, work. I agree it's not totally
obvious, and in any case there's going to be a point where the cast
happens where type safety will break, which is what I was alluding to in
my reply to Stanislav.

I guess we could try to rework the API in xdp_sock_drv.h to make this
more obvious instead of using the casting driver-specific wrappers I
suggested here. Or we could go with Stanislav's suggestion of keeping
allocation etc using xdp_buff and only casting to mlx5e_xdp_buff in the
function where it's used; then we can keep the casting localised to that
function, and put a comment there explaining why it works?

> Maybe letting mlx5e_xdp_buff point to its struct xdp_buff (instead of 
> wrapping it) will solve the problems here, then we'll loop over the 
> xdp_buff * array and copy the pointers into the struct mlx5e_xdp_buff * 
> array.
> Need to give it deeper thoughts...
>
> struct mlx5e_xdp_buff {
> 	struct xdp_buff *xdp;
> 	struct mlx5_cqe64 *cqe;
> 	struct mlx5e_rq *rq;
> };

This was actually my original proposal; we discussed this back on v2 of
this patch series. People generally felt that the 'cb' field approach
(originally suggested by Jakub) was better. See the discussion starting
from here:

https://lore.kernel.org/r/[email-masked]

-Toke