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tcp, udp: Don't include destination address in partially precomputed csums

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We partially prepopulate IP and TCP header structures including, amongst
other things the destination address, which for IPv4 is always the known
address of the guest/namespace.  We partially precompute both the IPv4
header checksum and the TCP checksum based on this.

In future we're going to want more flexibility with controlling the
destination for IPv4 (as we already do for IPv6), so this precomputed value
gets in the way.  Therefore remove the IPv4 destination from the
precomputed checksum and fold it into the checksum update when we actually
send a packet.

Doing this means we no longer need to recompute those partial sums when
the destination address changes ({tcp,udp}_update_l2_buf()) and instead
the computation can be moved to compile time.  This means while we perform
slightly more computations on each packet, we slightly reduce the amount of
memory we need to access.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
This commit is contained in:
David Gibson
2023-08-22 15:29:56 +10:00
committed by Stefano Brivio
parent 8aa32009ed
commit 5bf200ae8a
3 changed files with 27 additions and 52 deletions
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14
udp.c
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@@ -168,7 +168,6 @@ static uint8_t udp_act[IP_VERSIONS][UDP_ACT_TYPE_MAX][DIV_ROUND_UP(NUM_PORTS, 8)
/**
* udp4_l2_buf_t - Pre-cooked IPv4 packet buffers for tap connections
* @s_in: Source socket address, filled in by recvmmsg()
* @psum: Partial IP header checksum (excluding tot_len and saddr)
* @taph: Tap-level headers (partially pre-filled)
* @iph: Pre-filled IP header (except for tot_len and saddr)
* @uh: Headroom for UDP header
@@ -176,7 +175,6 @@ static uint8_t udp_act[IP_VERSIONS][UDP_ACT_TYPE_MAX][DIV_ROUND_UP(NUM_PORTS, 8)
*/
static struct udp4_l2_buf_t {
struct sockaddr_in s_in;
uint32_t psum;
struct tap_hdr taph;
struct iphdr iph;
@@ -263,11 +261,13 @@ static void udp_invert_portmap(struct udp_port_fwd *fwd)
*/
static void udp_update_check4(struct udp4_l2_buf_t *buf)
{
uint32_t sum = buf->psum;
uint32_t sum = L2_BUF_IP4_PSUM(IPPROTO_UDP);
sum += buf->iph.tot_len;
sum += (buf->iph.saddr >> 16) & 0xffff;
sum += buf->iph.saddr & 0xffff;
sum += (buf->iph.daddr >> 16) & 0xffff;
sum += buf->iph.daddr & 0xffff;
buf->iph.check = (uint16_t)~csum_fold(sum);
}
@@ -292,14 +292,6 @@ void udp_update_l2_buf(const unsigned char *eth_d, const unsigned char *eth_s,
if (ip_da) {
b4->iph.daddr = ip_da->s_addr;
if (!i) {
b4->iph.saddr = 0;
b4->iph.tot_len = 0;
b4->iph.check = 0;
b4->psum = sum_16b(&b4->iph, 20);
} else {
b4->psum = udp4_l2_buf[0].psum;
}
}
}
}