Re: [PATCH v5 05/12] arp: Pass queue pair explicitly through ARP send path

On Tue, Jun 16, 2026 at 02:51:23PM +0200, Laurent Vivier wrote:

Add a qpair parameter to arp(), arp_send_init_req(), and arp_announce(), forwarding it to tap_send_single() instead of hardcoding QPAIR_DEFAULT.

tap_start_connection() now consumes the qpair parameter it received in the previous commit.

fwd_neigh_table_update() has no queue pair context and keeps using QPAIR_DEFAULT.

No functional change.

Signed-off-by: Laurent Vivier --- arp.c | 15 +++++++++------ arp.h | 6 +++--- fwd.c | 2 +- tap.c | 6 ++---- 4 files changed, 15 insertions(+), 14 deletions(-)

diff --git a/arp.c b/arp.c index e97c4de86a99..1dc8b87cd993 100644 --- a/arp.c +++ b/arp.c @@ -63,11 +63,12 @@ static bool ignore_arp(const struct ctx *c, /** * arp() - Check if this is a supported ARP message, reply as needed * @c: Execution context + * @qpair: Queue pair on which to send the reply * @data: Single packet with Ethernet buffer * * Return: 1 if handled, -1 on failure */ -int arp(const struct ctx *c, struct iov_tail *data) +int arp(const struct ctx *c, unsigned int qpair, struct iov_tail *data)

The qpair parameter makes sense to me here: you want to send the reply on the same qpair the request came in on.

{ union inany_addr tgt; struct { @@ -112,7 +113,7 @@ int arp(const struct ctx *c, struct iov_tail *data) memcpy(resp.am.tha, am->sha, sizeof(resp.am.tha)); memcpy(resp.am.tip, am->sip, sizeof(resp.am.tip));

- tap_send_single(c, QPAIR_DEFAULT, &resp, sizeof(resp)); + tap_send_single(c, qpair, &resp, sizeof(resp));

return 1; } @@ -120,8 +121,9 @@ int arp(const struct ctx *c, struct iov_tail *data) /** * arp_send_init_req() - Send initial ARP request to retrieve guest MAC address * @c: Execution context + * @qpair: Queue pair on which to send the request */ -void arp_send_init_req(const struct ctx *c) +void arp_send_init_req(const struct ctx *c, unsigned int qpair)

But I'm dubious about it here. These are unsolicited messages to which we don't expect a reply. They're also low volume, so I don't see that we particularly need to distribute across queues. What's the advantage in putting QPAIR_DEFAULT in the caller, rather than right here?

{ struct { struct ethhdr eh; @@ -148,16 +150,17 @@ void arp_send_init_req(const struct ctx *c) memcpy(req.am.tip, &c->ip4.addr, sizeof(req.am.tip));

debug("Sending initial ARP request for guest MAC address"); - tap_send_single(c, QPAIR_DEFAULT, &req, sizeof(req)); + tap_send_single(c, qpair, &req, sizeof(req)); }

/** * arp_announce() - Send an ARP announcement for an IPv4 host * @c: Execution context + * @qpair: Queue pair on which to send the announcement * @ip: IPv4 address we announce as owned by @mac * @mac: MAC address to advertise for @ip */ -void arp_announce(const struct ctx *c, struct in_addr *ip, +void arp_announce(const struct ctx *c, unsigned int qpair, struct in_addr *ip, const unsigned char *mac)

Similar thoughts here.

{ char ip_str[INET_ADDRSTRLEN]; @@ -202,5 +205,5 @@ void arp_announce(const struct ctx *c, struct in_addr *ip, eth_ntop(mac, mac_str, sizeof(mac_str)); debug("ARP announcement for %s / %s", ip_str, mac_str);

- tap_send_single(c, QPAIR_DEFAULT, &msg, sizeof(msg)); + tap_send_single(c, qpair, &msg, sizeof(msg)); } diff --git a/arp.h b/arp.h index 4b1f38bcec9b..501760393a74 100644 --- a/arp.h +++ b/arp.h @@ -22,9 +22,9 @@ struct arpmsg { unsigned char tip[4]; } __attribute__((__packed__));

-int arp(const struct ctx *c, struct iov_tail *data); -void arp_send_init_req(const struct ctx *c); -void arp_announce(const struct ctx *c, struct in_addr *ip, +int arp(const struct ctx *c, unsigned int qpair, struct iov_tail *data); +void arp_send_init_req(const struct ctx *c, unsigned int qpair); +void arp_announce(const struct ctx *c, unsigned int qpair, struct in_addr *ip, const unsigned char *mac);

#endif /* ARP_H */ diff --git a/fwd.c b/fwd.c index c0a6adacd294..0d0e265b7dc0 100644 --- a/fwd.c +++ b/fwd.c @@ -145,7 +145,7 @@ void fwd_neigh_table_update(const struct ctx *c, const union inany_addr *addr, return;

if (inany_v4(addr)) - arp_announce(c, inany_v4(addr), e->mac); + arp_announce(c, QPAIR_DEFAULT, inany_v4(addr), e->mac); else ndp_unsolicited_na(c, &addr->a6); } diff --git a/tap.c b/tap.c index 521ccd6d47e7..66dcb83665a7 100644 --- a/tap.c +++ b/tap.c @@ -741,7 +741,7 @@ resume: if (!eh) continue; if (ntohs(eh->h_proto) == ETH_P_ARP) { - arp(c, &data); + arp(c, qpair, &data); continue; }

@@ -1453,8 +1453,6 @@ static void tap_start_connection(const struct ctx *c, unsigned int qpair) { union epoll_ref ref = { 0 };

- (void)qpair; - ref.fd = c->fd_tap; switch (c->mode) { case MODE_PASST: @@ -1474,7 +1472,7 @@ static void tap_start_connection(const struct ctx *c, unsigned int qpair) return;

if (c->ifi4) - arp_send_init_req(c); + arp_send_init_req(c, qpair); if (c->ifi6 && !c->no_ndp) ndp_send_init_req(c); } -- 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