linux-kernel - Re: [PATCH net-next 05/16] idpf: fix Rx descriptor ready check barrier in splitq

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Message-ID: <Z8rHp046ELv2wrac@boxer>
Date: Fri, 7 Mar 2025 11:17:11 +0100
From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
CC: <intel-wired-lan@...ts.osuosl.org>, Michal Kubiak
	<michal.kubiak@...el.com>, Tony Nguyen <anthony.l.nguyen@...el.com>, "Przemek
 Kitszel" <przemyslaw.kitszel@...el.com>, Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, "Alexei
 Starovoitov" <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
	"Jesper Dangaard Brouer" <hawk@...nel.org>, John Fastabend
	<john.fastabend@...il.com>, Simon Horman <horms@...nel.org>,
	<bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 05/16] idpf: fix Rx descriptor ready check
 barrier in splitq

On Wed, Mar 05, 2025 at 05:21:21PM +0100, Alexander Lobakin wrote:
> No idea what the current barrier position was meant for. At that point,
> nothing is read from the descriptor, only the pointer to the actual one
> is fetched.
> The correct barrier usage here is after the generation check, so that
> only the first qword is read if the descriptor is not yet ready and we
> need to stop polling. Debatable on coherent DMA as the Rx descriptor
> size is <= cacheline size, but anyway, the current barrier position
> only makes the codegen worse.

Makes sense:
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>

But you know the process... :P fixes should go to -net.

> 
> Fixes: 3a8845af66ed ("idpf: add RX splitq napi poll support")
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>
> ---
>  drivers/net/ethernet/intel/idpf/idpf_txrx.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/idpf/idpf_txrx.c b/drivers/net/ethernet/intel/idpf/idpf_txrx.c
> index 6254806c2072..c15833928ea1 100644
> --- a/drivers/net/ethernet/intel/idpf/idpf_txrx.c
> +++ b/drivers/net/ethernet/intel/idpf/idpf_txrx.c
> @@ -3232,18 +3232,14 @@ static int idpf_rx_splitq_clean(struct idpf_rx_queue *rxq, int budget)
>  		/* get the Rx desc from Rx queue based on 'next_to_clean' */
>  		rx_desc = &rxq->rx[ntc].flex_adv_nic_3_wb;
>  
> -		/* This memory barrier is needed to keep us from reading
> -		 * any other fields out of the rx_desc
> -		 */
> -		dma_rmb();
> -
>  		/* if the descriptor isn't done, no work yet to do */
>  		gen_id = le16_get_bits(rx_desc->pktlen_gen_bufq_id,
>  				       VIRTCHNL2_RX_FLEX_DESC_ADV_GEN_M);
> -
>  		if (idpf_queue_has(GEN_CHK, rxq) != gen_id)
>  			break;
>  
> +		dma_rmb();
> +
>  		rxdid = FIELD_GET(VIRTCHNL2_RX_FLEX_DESC_ADV_RXDID_M,
>  				  rx_desc->rxdid_ucast);
>  		if (rxdid != VIRTCHNL2_RXDID_2_FLEX_SPLITQ) {
> -- 
> 2.48.1
>