Re: [PATCH v2 02/10] tap: Make L4 sequence pools per-qpair for thread safety

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Jul 31, 2026 at 06:23:21PM +0200, Laurent Vivier wrote:
> The L4 sequence arrays tap4_l4[] and tap6_l4[] are used to batch
> packets with the same L4 tuple within a single tap_handler() call.
> They are global, but tap_handler() can be called concurrently from
> different worker threads with different qpairs in vhost-user mode.
> 
> Make these arrays per-qpair by adding a VHOST_USER_MAX_VQS/2 first
> dimension, indexed by the qpair parameter already available in
> tap4_handler() and tap6_handler().
> 
> Update tap_sock_update_pool() to initialize all qpair*seq entries.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  tap.c | 32 +++++++++++++++++++-------------
>  1 file changed, 19 insertions(+), 13 deletions(-)
> 
> diff --git a/tap.c b/tap.c
> index 6bba436f148f..25cde67538ac 100644
> --- a/tap.c
> +++ b/tap.c
> @@ -591,7 +591,7 @@ static struct tap4_l4_t {
>  	struct in_addr daddr;
>  
>  	struct pool_l4_t p;
> -} tap4_l4[TAP_SEQS /* Arbitrary: TAP_MSGS in theory, so limit in users */];
> +} tap4_l4[VHOST_USER_MAX_VQS / 2][TAP_SEQS /* Arbitrary: TAP_MSGS in theory, so limit in users */];
>  
>  /**
>   * struct l4_seq6_t - Message sequence for one protocol handler call, IPv6
> @@ -618,7 +618,7 @@ static struct tap6_l4_t {
>  	uint8_t hop_limit;
>  
>  	struct pool_l4_t p;
> -} tap6_l4[TAP_SEQS /* Arbitrary: TAP_MSGS in theory, so limit in users */];
> +} tap6_l4[VHOST_USER_MAX_VQS / 2][TAP_SEQS /* Arbitrary: TAP_MSGS in theory, so limit in users */];
>  
>  /**
>   * tap_packet_debug() - Print debug message for packet(s) from guest/tap
> @@ -838,7 +838,7 @@ resume:
>  		if (seq_count == TAP_SEQS)
>  			break;	/* Resume after flushing if i < pool_tap4[qpair]->count */
>  
> -		for (seq = tap4_l4 + seq_count - 1; seq >= tap4_l4; seq--) {
> +		for (seq = tap4_l4[qpair] + seq_count - 1; seq >= tap4_l4[qpair]; seq--) {
>  			if (L4_MATCH(iph, uh, seq)) {
>  				if (seq->p.count >= UIO_MAXIOV)
>  					seq = NULL;
> @@ -846,8 +846,8 @@ resume:
>  			}
>  		}
>  
> -		if (!seq || seq < tap4_l4) {
> -			seq = tap4_l4 + seq_count++;
> +		if (!seq || seq < tap4_l4[qpair]) {
> +			seq = tap4_l4[qpair] + seq_count++;
>  			L4_SET(iph, uh, seq);
>  			pool_flush((struct pool *)&seq->p);
>  		}
> @@ -859,7 +859,7 @@ append:
>  		packet_add((struct pool *)&seq->p, &data);
>  	}
>  
> -	for (j = 0, seq = tap4_l4; j < seq_count; j++, seq++) {
> +	for (j = 0, seq = tap4_l4[qpair]; j < seq_count; j++, seq++) {
>  		const struct pool *p = (const struct pool *)&seq->p;
>  		size_t k;
>  
> @@ -1088,7 +1088,7 @@ resume:
>  		if (seq_count == TAP_SEQS)
>  			break;	/* Resume after flushing if i < pool_tap6[qpair]->count */
>  
> -		for (seq = tap6_l4 + seq_count - 1; seq >= tap6_l4; seq--) {
> +		for (seq = tap6_l4[qpair] + seq_count - 1; seq >= tap6_l4[qpair]; seq--) {
>  			if (L4_MATCH(ip6h, proto, uh, seq)) {
>  				if (seq->p.count >= UIO_MAXIOV)
>  					seq = NULL;
> @@ -1096,8 +1096,8 @@ resume:
>  			}
>  		}
>  
> -		if (!seq || seq < tap6_l4) {
> -			seq = tap6_l4 + seq_count++;
> +		if (!seq || seq < tap6_l4[qpair]) {
> +			seq = tap6_l4[qpair] + seq_count++;
>  			L4_SET(ip6h, proto, uh, seq);
>  			pool_flush((struct pool *)&seq->p);
>  		}
> @@ -1109,7 +1109,7 @@ append:
>  		packet_add((struct pool *)&seq->p, &data);
>  	}
>  
> -	for (j = 0, seq = tap6_l4; j < seq_count; j++, seq++) {
> +	for (j = 0, seq = tap6_l4[qpair]; j < seq_count; j++, seq++) {
>  		const struct pool *p = (const struct pool *)&seq->p;
>  		size_t k;
>  
> @@ -1602,9 +1602,15 @@ static void tap_sock_update_pool(void *base, size_t size)
>  		pool_tap6[i] = (struct pool *)&pool_tap6_storage[i];
>  	}
>  
> -	for (i = 0; i < TAP_SEQS; i++) {
> -		tap4_l4[i].p = PACKET_INIT(pool_l4, UIO_MAXIOV, base, size);
> -		tap6_l4[i].p = PACKET_INIT(pool_l4, UIO_MAXIOV, base, size);
> +	for (i = 0; i < VHOST_USER_MAX_VQS / 2; i++) {
> +		unsigned int j;
> +
> +		for (j = 0; j < TAP_SEQS; j++) {
> +			tap4_l4[i][j].p = PACKET_INIT(pool_l4, UIO_MAXIOV,
> +						      base, size);
> +			tap6_l4[i][j].p = PACKET_INIT(pool_l4, UIO_MAXIOV,
> +						      base, size);
> +		}
>  	}
>  }
>  
> -- 
> 2.54.0
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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