netdev - Re: [PATCH net] ice: xsk: clear status

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Date:   Fri, 26 Nov 2021 14:39:54 +0100
From:   Alexander Lobakin <alexandr.lobakin@...el.com>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        netdev@...r.kernel.org, kuba@...nel.org, davem@...emloft.net,
        bpf@...r.kernel.org, anthony.l.nguyen@...el.com,
        magnus.karlsson@...el.com
Subject: Re: [PATCH net] ice: xsk: clear status_error0 for each allocated desc

From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Date: Fri, 26 Nov 2021 15:38:48 +0100

> Fix a bug in which the receiving of packets can stop in the zero-copy
> driver. Ice HW ignores 3 lower bits from QRX_TAIL register, which means
> that tail is bumped only on intervals of 8. Currently with XSK RX
> batching in place, ice_alloc_rx_bufs_zc() clears the status_error0 only
> of the last descriptor that has been allocated/taken from the XSK buffer
> pool. status_error0 includes DD bit that is looked upon by the
> ice_clean_rx_irq_zc() to tell if a descriptor can be processed.
> 
> The bug can be triggered when driver updates the ntu but not the
> QRX_TAIL, so HW wouldn't have a chance to write to the ready
> descriptors. Later on driver moves the ntc to the mentioned set of
> descriptors and interprets them as a ready to be processed, since
> corresponding DD bits were not cleared nor any writeback has happened
> that would clear it. This can then lead to ntc == ntu case which means
> that ring is empty and no further packet processing.
> 
> Fix the XSK traffic hang that can be observed when l2fwd scenario from
> xdpsock is used by making sure that status_error0 is cleared for each
> descriptor that is fed to HW and therefore we are sure that driver will
> not processed non-valid DD bits. This will also prevent the driver from
> processing the descriptors that were allocated in favor of the
> previously processed ones, but writeback didn't happen yet.
> 
> Fixes: db804cfc21e9 ("ice: Use the xsk batched rx allocation interface")
> Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>

Reviewed-by: Alexander Lobakin <alexandr.lobakin@...el.com>

Thanks!

> ---
>  drivers/net/ethernet/intel/ice/ice_xsk.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/net/ethernet/intel/ice/ice_xsk.c b/drivers/net/ethernet/intel/ice/ice_xsk.c
> index ff55cb415b11..bb9a80847298 100644
> --- a/drivers/net/ethernet/intel/ice/ice_xsk.c
> +++ b/drivers/net/ethernet/intel/ice/ice_xsk.c
> @@ -383,6 +383,7 @@ bool ice_alloc_rx_bufs_zc(struct ice_rx_ring *rx_ring, u16 count)
>  	while (i--) {
>  		dma = xsk_buff_xdp_get_dma(*xdp);
>  		rx_desc->read.pkt_addr = cpu_to_le64(dma);
> +		rx_desc->wb.status_error0 = 0;
>  
>  		rx_desc++;
>  		xdp++;
> -- 
> 2.31.1

Al