netdev - Re: [PATCH net] gve: Fix XDP TX completion handling when counters overflow

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Message-ID: <20240717074122.GH249423@kernel.org>
Date: Wed, 17 Jul 2024 08:41:22 +0100
From: Simon Horman <horms@...nel.org>
To: Praveen Kaligineedi <pkaligineedi@...gle.com>
Cc: netdev@...r.kernel.org, stable@...nel.org, hramamurthy@...gle.com,
	jfraker@...gle.com, jeroendb@...gle.com, shailend@...gle.com,
	davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, ast@...nel.org, daniel@...earbox.net,
	hawk@...nel.org, john.fastabend@...il.com, ziweixiao@...gle.com,
	willemb@...gle.com, Joshua Washington <joshwash@...gle.com>
Subject: Re: [PATCH net] gve: Fix XDP TX completion handling when counters
 overflow

On Tue, Jul 16, 2024 at 10:10:41AM -0700, Praveen Kaligineedi wrote:
> From: Joshua Washington <joshwash@...gle.com>
> 
> In gve_clean_xdp_done, the driver processes the TX completions based on
> a 32-bit NIC counter and a 32-bit completion counter stored in the tx
> queue.
> 
> Fix the for loop so that the counter wraparound is handled correctly.
> 
> Fixes: 75eaae158b1b ("gve: Add XDP DROP and TX support for GQI-QPL format")
> Signed-off-by: Joshua Washington <joshwash@...gle.com>
> Signed-off-by: Praveen Kaligineedi <pkaligineedi@...gle.com>

Reviewed-by: Simon Horman <horms@...nel.org>

> ---
>  drivers/net/ethernet/google/gve/gve_tx.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/google/gve/gve_tx.c b/drivers/net/ethernet/google/gve/gve_tx.c
> index 24a64ec1073e..e7fb7d6d283d 100644
> --- a/drivers/net/ethernet/google/gve/gve_tx.c
> +++ b/drivers/net/ethernet/google/gve/gve_tx.c
> @@ -158,15 +158,16 @@ static int gve_clean_xdp_done(struct gve_priv *priv, struct gve_tx_ring *tx,
>  			      u32 to_do)
>  {
>  	struct gve_tx_buffer_state *info;
> -	u32 clean_end = tx->done + to_do;
>  	u64 pkts = 0, bytes = 0;
>  	size_t space_freed = 0;
>  	u32 xsk_complete = 0;
>  	u32 idx;
> +	int i;
>  
> -	for (; tx->done < clean_end; tx->done++) {
> +	for (i = 0; i < to_do; i++) {

I was slightly concerned that, as it is a u32, the value of to_do could
exceed the maximum value of i, which is an int.

But I see that in practice the value of to_do is bound by an int, budget,
in the call site, gve_xdp_poll. So I think we are ok.

Perhaps, as a clean up, the type of the to_do parameter of
gve_clean_xdp_done, and local variable in gve_xdp_poll() could be updated
from u32 to int. But, OTOH, perhaps that doesn't get us anywhere.

>  		idx = tx->done & tx->mask;
>  		info = &tx->info[idx];
> +		tx->done++;
>  
>  		if (unlikely(!info->xdp.size))
>  			continue;
> -- 
> 2.45.2.803.g4e1b14247a-goog
> 
>