Re: [PATCH v6 1/4] tcp: Encode checksum computation flags in a single parameter

On Thu, Apr 16, 2026 at 06:16:15PM +0200, Laurent Vivier wrote:

tcp_fill_headers() takes a pointer to a previously computed IPv4 header checksum to avoid recalculating it when the payload length doesn't change, and a separate bool to skip TCP checksum computation.

Replace both parameters with a single uint32_t csum_flags that encodes: - IP4_CSUM (bit 31): compute IPv4 header checksum from scratch - TCP_CSUM (bit 30): compute TCP checksum - IP4_CMASK (low 16 bits): cached IPv4 header checksum value

When IP4_CSUM is not set, the cached checksum is extracted from the low 16 bits. This is cleaner than the pointer-based approach, and also avoids a potential dangling pointer issue: a subsequent patch makes tcp_fill_headers() access ip4h via with_header(), which scopes it to a temporary variable, so a pointer to ip4h->check would become invalid after the with_header() block.

Suggested-by: David Gibson Signed-off-by: Laurent Vivier

Reviewed-by: David Gibson

--- tcp.c | 25 +++++++++++++------------ tcp_buf.c | 23 ++++++++++++----------- tcp_internal.h | 7 +++++-- tcp_vu.c | 28 +++++++++++++++++----------- 4 files changed, 47 insertions(+), 36 deletions(-)

diff --git a/tcp.c b/tcp.c index 45bcc19375fe..de362290b034 100644 --- a/tcp.c +++ b/tcp.c @@ -946,9 +946,10 @@ static void tcp_fill_header(struct tcphdr *th, * @th: Pointer to TCP header * @payload: TCP payload * @dlen: TCP payload length - * @ip4_check: IPv4 checksum, if already known + * @csum_flags: TCP_CSUM if TCP checksum must be computed, + * IP4_CSUM if IPv4 checksum must be computed, + * otherwise IPv4 checksum is provided in IP4_CMASK * @seq: Sequence number for this segment - * @no_tcp_csum: Do not set TCP checksum * * Return: frame length (including L2 headers) */ @@ -956,8 +957,7 @@ size_t tcp_fill_headers(const struct ctx *c, struct tcp_tap_conn *conn, struct ethhdr *eh, struct iphdr *ip4h, struct ipv6hdr *ip6h, struct tcphdr *th, struct iov_tail *payload, - size_t dlen, const uint16_t *ip4_check, uint32_t seq, - bool no_tcp_csum) + size_t dlen, uint32_t csum_flags, uint32_t seq) { const struct flowside *tapside = TAPFLOW(conn); size_t l4len = dlen + sizeof(*th); @@ -977,13 +977,14 @@ size_t tcp_fill_headers(const struct ctx *c, struct tcp_tap_conn *conn, ip4h->saddr = src4->s_addr; ip4h->daddr = dst4->s_addr;

- if (ip4_check) - ip4h->check = *ip4_check; - else + if (csum_flags & IP4_CSUM) { ip4h->check = csum_ip4_header(l3len, IPPROTO_TCP, *src4, *dst4); + } else { + ip4h->check = csum_flags & IP4_CMASK; + }

- if (!no_tcp_csum) { + if (csum_flags & TCP_CSUM) { psum = proto_ipv4_header_psum(l4len, IPPROTO_TCP, *src4, *dst4); } @@ -1003,7 +1004,7 @@ size_t tcp_fill_headers(const struct ctx *c, struct tcp_tap_conn *conn,

ip6_set_flow_lbl(ip6h, conn->sock);

- if (!no_tcp_csum) { + if (csum_flags & TCP_CSUM) { psum = proto_ipv6_header_psum(l4len, IPPROTO_TCP, &ip6h->saddr, &ip6h->daddr); @@ -1018,10 +1019,10 @@ size_t tcp_fill_headers(const struct ctx *c, struct tcp_tap_conn *conn,

tcp_fill_header(th, conn, seq);

- if (no_tcp_csum) - th->check = 0; - else + if (csum_flags & TCP_CSUM) tcp_update_csum(psum, th, payload, dlen); + else + th->check = 0;

return MAX(l3len + sizeof(struct ethhdr), ETH_ZLEN); } diff --git a/tcp_buf.c b/tcp_buf.c index 27151854033c..a27d9733616c 100644 --- a/tcp_buf.c +++ b/tcp_buf.c @@ -166,14 +166,15 @@ static void tcp_l2_buf_pad(struct iovec *iov) * @c: Execution context * @conn: Connection pointer * @iov: Pointer to an array of iovec of TCP pre-cooked buffers - * @check: Checksum, if already known + * @csum_flags: TCP_CSUM if TCP checksum must be computed, + * IP4_CSUM if IPv4 checksum must be computed, + * otherwise IPv4 checksum is provided in IP4_CMASK * @seq: Sequence number for this segment - * @no_tcp_csum: Do not set TCP checksum */ static void tcp_l2_buf_fill_headers(const struct ctx *c, struct tcp_tap_conn *conn, - struct iovec *iov, const uint16_t *check, - uint32_t seq, bool no_tcp_csum) + struct iovec *iov, uint32_t csum_flags, + uint32_t seq) { struct iov_tail tail = IOV_TAIL(&iov[TCP_IOV_PAYLOAD], 1, 0); struct tcphdr th_storage, *th = IOV_REMOVE_HEADER(&tail, th_storage); @@ -191,8 +192,7 @@ static void tcp_l2_buf_fill_headers(const struct ctx *c, ip6h = iov[TCP_IOV_IP].iov_base;

l2len = tcp_fill_headers(c, conn, eh, ip4h, ip6h, th, &tail, - iov_tail_size(&tail), check, seq, - no_tcp_csum); + iov_tail_size(&tail), csum_flags, seq); tap_hdr_update(taph, l2len); }

@@ -234,7 +234,7 @@ int tcp_buf_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags) if (flags & KEEPALIVE) seq--;

- tcp_l2_buf_fill_headers(c, conn, iov, NULL, seq, false); + tcp_l2_buf_fill_headers(c, conn, iov, IP4_CSUM | TCP_CSUM, seq);

tcp_l2_buf_pad(iov);

@@ -271,7 +271,7 @@ static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn, ssize_t dlen, int no_csum, uint32_t seq, bool push) { struct tcp_payload_t *payload; - const uint16_t *check = NULL; + uint32_t check = IP4_CSUM; struct iovec *iov;

conn->seq_to_tap = seq + dlen; @@ -280,9 +280,10 @@ static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn, if (CONN_V4(conn)) { if (no_csum) { struct iovec *iov_prev = tcp_l2_iov[tcp_payload_used - 1]; - struct iphdr *iph = iov_prev[TCP_IOV_IP].iov_base; + const struct iphdr *iph = iov_prev[TCP_IOV_IP].iov_base;

- check = &iph->check; + /* overwrite IP4_CSUM flag as we set the checksum */ + check = iph->check; } iov[TCP_IOV_IP] = IOV_OF_LVALUE(tcp4_payload_ip[tcp_payload_used]); } else if (CONN_V6(conn)) { @@ -296,7 +297,7 @@ static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn, payload->th.ack = 1; payload->th.psh = push; iov[TCP_IOV_PAYLOAD].iov_len = dlen + sizeof(struct tcphdr); - tcp_l2_buf_fill_headers(c, conn, iov, check, seq, false); + tcp_l2_buf_fill_headers(c, conn, iov, TCP_CSUM | check, seq);

tcp_l2_buf_pad(iov);

diff --git a/tcp_internal.h b/tcp_internal.h index a0fa19f4ed11..40472c9973c8 100644 --- a/tcp_internal.h +++ b/tcp_internal.h @@ -183,12 +183,15 @@ void tcp_rst_do(const struct ctx *c, struct tcp_tap_conn *conn);

struct tcp_info_linux;

+#define IP4_CSUM 0x80000000 +#define IP4_CMASK 0x0000FFFF +#define TCP_CSUM 0x40000000 + size_t tcp_fill_headers(const struct ctx *c, struct tcp_tap_conn *conn, struct ethhdr *eh, struct iphdr *ip4h, struct ipv6hdr *ip6h, struct tcphdr *th, struct iov_tail *payload, - size_t dlen, const uint16_t *ip4_check, uint32_t seq, - bool no_tcp_csum); + size_t dlen, uint32_t csum_flags, uint32_t seq);

int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn, bool force_seq, struct tcp_info_linux *tinfo); diff --git a/tcp_vu.c b/tcp_vu.c index 2dfe14485eee..3e399c20f0d7 100644 --- a/tcp_vu.c +++ b/tcp_vu.c @@ -134,7 +134,7 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags) seq--;

tcp_fill_headers(c, conn, eh, ip4h, ip6h, th, &payload, - optlen, NULL, seq, !*c->pcap); + optlen, IP4_CSUM | (*c->pcap ? TCP_CSUM : 0), seq);

vu_pad(flags_elem[0].in_sg, 1, hdrlen + optlen); vu_flush(vdev, vq, flags_elem, 1, hdrlen + optlen); @@ -282,13 +282,15 @@ static ssize_t tcp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, * @iov: Pointer to the array of IO vectors * @iov_cnt: Number of entries in @iov * @dlen: Data length - * @check: Checksum, if already known - * @no_tcp_csum: Do not set TCP checksum + * @csum_flags: Pointer to checksum flags (input/output) + * TCP_CSUM if TCP checksum must be computed, + * IP4_CSUM if IPv4 checksum must be computed, + * otherwise IPv4 checksum is provided in IP4_CMASK * @push: Set PSH flag, last segment in a batch */ static void tcp_vu_prepare(const struct ctx *c, struct tcp_tap_conn *conn, struct iovec *iov, size_t iov_cnt, size_t dlen, - const uint16_t **check, bool no_tcp_csum, bool push) + uint32_t *csum_flags, bool push) { const struct flowside *toside = TAPFLOW(conn); bool v6 = !(inany_v4(&toside->eaddr) && inany_v4(&toside->oaddr)); @@ -332,9 +334,11 @@ static void tcp_vu_prepare(const struct ctx *c, struct tcp_tap_conn *conn, th->psh = push;

tcp_fill_headers(c, conn, eh, ip4h, ip6h, th, &payload, dlen, - *check, conn->seq_to_tap, no_tcp_csum); + *csum_flags, conn->seq_to_tap); + + /* Preserve TCP_CSUM, overwrite IP4_CSUM as we set the checksum */ if (ip4h) - *check = &ip4h->check; + *csum_flags = (*csum_flags & TCP_CSUM) | ip4h->check; }

/** @@ -350,12 +354,11 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn) uint32_t wnd_scaled = conn->wnd_from_tap << conn->ws_from_tap; struct vu_dev *vdev = c->vdev; struct vu_virtq *vq = &vdev->vq[VHOST_USER_RX_QUEUE]; + uint32_t already_sent, check; ssize_t len, previous_dlen; int i, iov_cnt, head_cnt; size_t hdrlen, fillsize; int v6 = CONN_V6(conn); - uint32_t already_sent; - const uint16_t *check;

if (!vu_queue_enabled(vq) || !vu_queue_started(vq)) { debug("Got packet, but RX virtqueue not usable yet"); @@ -442,7 +445,10 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn) */

hdrlen = tcp_vu_hdrlen(v6); - for (i = 0, previous_dlen = -1, check = NULL; i < head_cnt; i++) { + check = IP4_CSUM; + if (*c->pcap) + check |= TCP_CSUM; + for (i = 0, previous_dlen = -1; i < head_cnt; i++) { struct iovec *iov = &elem[head[i]].in_sg[0]; int buf_cnt = head[i + 1] - head[i]; size_t frame_size = iov_size(iov, buf_cnt); @@ -458,10 +464,10 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)

/* The IPv4 header checksum varies only with dlen */ if (previous_dlen != dlen) - check = NULL; + check |= IP4_CSUM; previous_dlen = dlen;

- tcp_vu_prepare(c, conn, iov, buf_cnt, dlen, &check, !*c->pcap, push); + tcp_vu_prepare(c, conn, iov, buf_cnt, dlen, &check, push);

vu_pad(elem[head[i]].in_sg, buf_cnt, dlen + hdrlen); vu_flush(vdev, vq, &elem[head[i]], buf_cnt, dlen + hdrlen); -- 2.53.0

-- 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