mirrors/passt
1
0
Fork 0
mirror of https://passt.top/passt synced 2025-05-25 10:35:35 +02:00
Code Issues Projects Releases Packages Wiki Activity Actions

tap: Clarify calculation of TAP_MSGS

Browse source 
The rationale behind the calculation of TAP_MSGS isn't necessarily obvious.
It's supposed to be the maximum number of packets that can fit in pkt_buf.
However, the calculation is wrong in several ways:
 * It's based on ETH_ZLEN which isn't meaningful for virtual devices
 * It always includes the qemu socket header which isn't used for pasta
 * The size of pkt_buf isn't relevant for vhost-user

We've already made sure this is just a tuning parameter, not a hard limit.
Clarify what we're calculating here and why.

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 2025-03-17 20:24:17 +11:00 • committed by Stefano Brivio
parent a41d6d125e
commit 9866d146e6
1 changed files with 22 additions and 6 deletions

28
tap.c
View file

@ -75,12 +75,28 @@ CHECK_FRAME_LEN(L2_MAX_LEN_PASTA);
CHECK_FRAME_LEN(L2_MAX_LEN_PASST);
CHECK_FRAME_LEN(L2_MAX_LEN_VU);
#define TAP_MSGS \
DIV_ROUND_UP(sizeof(pkt_buf), ETH_ZLEN - 2 * ETH_ALEN + sizeof(uint32_t))
/* We try size the packet pools so that we can use a single batch for the entire
* packet buffer. This might be exceeded for vhost-user, though, which uses its
* own buffers rather than pkt_buf.
*
* This is just a tuning parameter, the code will work with slightly more
* overhead if it's incorrect. So, we estimate based on the minimum practical
* frame size - an empty UDP datagram - rather than the minimum theoretical
* frame size.
*
* FIXME: Profile to work out how big this actually needs to be to amortise
* per-batch syscall overheads
*/
#define TAP_MSGS_IP4 \
DIV_ROUND_UP(sizeof(pkt_buf), \
ETH_HLEN + sizeof(struct iphdr) + sizeof(struct udphdr))
#define TAP_MSGS_IP6 \
DIV_ROUND_UP(sizeof(pkt_buf), \
ETH_HLEN + sizeof(struct ipv6hdr) + sizeof(struct udphdr))
/* IPv4 (plus ARP) and IPv6 message batches from tap/guest to IP handlers */
static PACKET_POOL_NOINIT(pool_tap4, TAP_MSGS, pkt_buf);
static PACKET_POOL_NOINIT(pool_tap6, TAP_MSGS, pkt_buf);
static PACKET_POOL_NOINIT(pool_tap4, TAP_MSGS_IP4, pkt_buf);
static PACKET_POOL_NOINIT(pool_tap6, TAP_MSGS_IP6, pkt_buf);
#define TAP_SEQS 128 /* Different L4 tuples in one batch */
#define FRAGMENT_MSG_RATE 10 /* # seconds between fragment warnings */
@ -1418,8 +1434,8 @@ void tap_sock_update_pool(void *base, size_t size)
{
int i;
pool_tap4_storage = PACKET_INIT(pool_tap4, TAP_MSGS, base, size);
pool_tap6_storage = PACKET_INIT(pool_tap6, TAP_MSGS, base, size);
pool_tap4_storage = PACKET_INIT(pool_tap4, TAP_MSGS_IP4, base, size);
pool_tap6_storage = PACKET_INIT(pool_tap6, TAP_MSGS_IP6, base, size);
for (i = 0; i < TAP_SEQS; i++) {
tap4_l4[i].p = PACKET_INIT(pool_l4, UIO_MAXIOV, base, size);