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

[Simon Horman <horms@kernel.org>]

On Thu, Mar 27, 2025 at 07:58:30PM +0200, Tariq Toukan wrote:
> 
> 
> 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().

Hi Tariq,

Sorry for the slow response, I was AFK for a few days.

Thanks for confirming that this is as expected.