Re: [PATCH net] net/mlx5e: SHAMPO, Make reserved size independent of page size

[Tariq Toukan <ttoukan.linux@gmail.com>]

On 25/03/2025 16:04, Simon Horman wrote:
> On Sun, Mar 23, 2025 at 02:28:26PM +0200, Tariq Toukan wrote:
>> From: Lama Kayal <lkayal@nvidia.com>
>>
>> When hw-gro is enabled, the maximum number of header entries that are
>> needed per wqe (hd_per_wqe) is calculated based on the size of the
>> reservations among other parameters.
>>
>> Miscalculation of the size of reservations leads to incorrect
>> calculation of hd_per_wqe as 0, particularly in the case of large page
>> size like in aarch64, this prevents the SHAMPO header from being
>> correctly initialized in the device, ultimately causing the following
>> cqe err that indicates a violation of PD.
> 
> Hi Lama, Tariq, all,
> 
> If I understand things correctly, hd_per_wqe is calculated
> in mlx5e_shampo_hd_per_wqe() like this:
> 
> u32 mlx5e_shampo_hd_per_wqe(struct mlx5_core_dev *mdev,
>                              struct mlx5e_params *params,                                                    struct mlx5e_rq_param *rq_param)
> {
>          int resv_size = BIT(mlx5e_shampo_get_log_rsrv_size(mdev, params)) * PAGE_SIZE;
>          u16 num_strides = BIT(mlx5e_mpwqe_get_log_num_strides(mdev, params, NULL));
>          int pkt_per_resv = BIT(mlx5e_shampo_get_log_pkt_per_rsrv(mdev, params));
>          u8 log_stride_sz = mlx5e_mpwqe_get_log_stride_size(mdev, params, NULL);
>          int wqe_size = BIT(log_stride_sz) * num_strides;                                u32 hd_per_wqe;
> 
>          /* Assumption: hd_per_wqe % 8 == 0. */
>          hd_per_wqe = (wqe_size / resv_size) * pkt_per_resv;                             mlx5_core_dbg(mdev, "%s hd_per_wqe = %d rsrv_size = %d wqe_size = %d pkt_per_resv = %d\n",                                                                                    __func__, hd_per_wqe, resv_size, wqe_size, pkt_per_resv);
>          return hd_per_wqe;
> }
> 
> I can see that if PAGE_SIZE was some multiple of 4k, and thus
> larger than wqe_size, then this could lead to hd_per_wqe being zero.
> 
> But I note that mlx5e_mpwqe_get_log_stride_size() may return PAGE_SHIFT.
> And I wonder if that leads to wqe_size being larger than expected by this
> patch in cases where the PAGE_SIZE is greater than 4k.
> 
> Likewise in mlx5e_shampo_get_log_cq_size(), which seems to have a large overlap
> codewise with mlx5e_shampo_hd_per_wqe().
> 
>>

Hi Simon,

Different settings lead to different combinations of num_strides and 
stride_size. However, they affect each other in a way that the resulting 
wqe_size has the expected (~preset) value.

In mlx5e_mpwqe_get_log_num_strides() you can see that if stride_size 
grows, then num_strides decreases accordingly.

In addition, to reduce mistakes/bugs, we have a few WARNs() along the 
calculations, in addition to a verifier function 
mlx5e_verify_rx_mpwqe_strides().

Thanks,
Tariq