Re: [PATCH v2 4/7] tap: Implement the pasta vhost-net from-guest path

On 10/16/26 22:49, aerosouund wrote:

From: Ammar Yasser

Add tap_vhost_input(), called when the kernel signals on a call eventfd that it has written guest frames into the from-guest queue. It consumes descriptors with consume_one_rx_descriptor(), skips the virtio_net header on each, builds an iov_tail from what's left and queues it for processing, then tells the kernel the descriptors are free to reuse.

consume_one_rx_descriptor() pops one entry from the used ring, advances our read cursor and the count of descriptors awaiting handoff, and returns a pointer to that descriptor's data.

Bootstrap vhost in tap_sock_tun_init(). If setup fails we leave c->vhost.fd at -1 and carry on unaccelerated.

With vhost-net running the kernel reads the tap fd itself, so don't add that fd to our epoll set at all: tap_handler_pasta() would otherwise be woken for data it must not read.

Signed-off-by: Eugenio Pérez Signed-off-by: Ammar Yasser --- passt.c | 6 +++ tap.c | 112 +++++++++++++++++++++++++++++++++++++++++++++++++++++++- tap.h | 2 + 3 files changed, 119 insertions(+), 1 deletion(-)

diff --git a/passt.c b/passt.c index 6002231..372b743 100644 --- a/passt.c +++ b/passt.c @@ -305,6 +305,12 @@ static void passt_worker(void *opaque, int nfds, struct epoll_event *events) case EPOLL_TYPE_CONF: conf_handler(c, eventmask); break; + case EPOLL_TYPE_VHOST_CALL: + tap_vhost_input(c, ref, &now); + break; + case EPOLL_TYPE_VHOST_ERROR: + die("Error on vhost-kernel socket"); + break; default: /* Can't happen */ assert(0); diff --git a/tap.c b/tap.c index dfa66c7..a5fdee8 100644 --- a/tap.c +++ b/tap.c @@ -38,6 +38,7 @@ #include #include #include +#include

#include #include @@ -61,6 +62,7 @@ #include "vhost_user.h" #include "vu_common.h" #include "epoll_ctl.h" +#include "vhost.h"

/* Maximum allowed frame lengths (including L2 header) */

@@ -1446,7 +1448,11 @@ static void tap_start_connection(const struct ctx *c) break; }

- epoll_add(c->epollfd, EPOLLIN | EPOLLRDHUP, ref); + /* With vhost-net running, the kernel reads the tap fd for us and + * notifies us on the call eventfd, so we mustn't watch it ourselves + */ + if (c->vhost.fd == -1) + epoll_add(c->epollfd, EPOLLIN | EPOLLRDHUP, ref);

if (!tap_is_ready(c)) return; @@ -1514,6 +1520,89 @@ void tap_listen_handler(struct ctx *c, uint32_t events) tap_start_connection(c); }

+/** + * consume_one_rx_descriptor() - Consume one used from-guest descriptor + * @len: Set to the length of data written by the kernel + * + * Pops a single entry from the used ring, advancing our read cursor and the + * count of descriptors awaiting refill announcement. + * + * NOTE: This function assumes the kernel is going to post single descriptors + * always, No chains. If that changes, we would need to increment num_free + * as we advertise back to the kernel the free descriptors by the length of the chain. + * + * Return: pointer to the packet buffer, or NULL if no data is available + */ +static void *consume_one_rx_descriptor(size_t *len)

pass the queue_idx

+{ + struct vring_used_pasta *used = &vring_used_all[VHOST_AVAIL_Q_IDX];

use queue_idx rather than VHOST_AVAIL_Q_IDX

+ uint16_t used_idx, last_used; + uint32_t i; + + used_idx = le16toh(used->idx); + + smp_rmb(); + + /* If the kernel's used index matches ours, we've consumed everything + * it has posted + */ + if (used_idx == vhost_vq_state[VHOST_AVAIL_Q_IDX].last_used_idx) { + *len = 0; + return NULL; + } + + last_used = vhost_vq_state[VHOST_AVAIL_Q_IDX].last_used_idx % VHOST_NDESCS;

VHOST_NDESCS should be repaced by vhost_vq_state[queue_idx].num_free

+ i = le32toh(used->ring[last_used].id); + *len = le32toh(used->ring[last_used].len); + + if (i != last_used) + die("vhost: id %u at used position %u", i, last_used); + + /* The kernel has queued for us something we cannot receive */ + if (*len > VHOST_DESC_BYTES) + die("vhost: id %u len %zu > %zu", i, *len, + (size_t)VHOST_DESC_BYTES); + + vhost_vq_state[VHOST_AVAIL_Q_IDX].last_used_idx++; + vhost_vq_state[VHOST_AVAIL_Q_IDX].num_free++; + + return pkt_buf + i * VHOST_DESC_BYTES; +} + +/** + * tap_vhost_input() - Handle frames the kernel wrote to the from-guest queue + * @c: Execution context + * @ref: epoll reference + * @now: Current timestamp + */ +void tap_vhost_input(struct ctx *c, union epoll_ref ref, + const struct timespec *now) +{ + struct virtio_net_hdr_mrg_rxbuf *hdr; + struct iov_tail data; + size_t len; + + eventfd_read(ref.fd, (eventfd_t[]){ 0 }); + + tap_flush_pools(); +

In ref.queue we should have queue_idx we can pass to consume_one_rx_descriptor()

+ while ((hdr = consume_one_rx_descriptor(&len))) { + /* a valid receive is one where there is a virtio net header followed by atleast sizeof(struct ethhdr) bytes */ + if (len < (sizeof(*hdr) + sizeof(struct ethhdr))) { + warn("vhost: invalid len %lu", len); + continue; + } + + /* Skip over the vnet header, add the packet without it */ + data = IOV_TAIL_FROM_BUF((void *)(hdr + 1), + len - sizeof(*hdr), 0); + tap_add_packet(c, &data, now); + } + + tap_handler(c, now); + vhost_rx_descriptor_handoff(c); +} + /** * tap_ns_tun() - Get tuntap fd in namespace * @c: Execution context @@ -1555,12 +1644,33 @@ static int tap_ns_tun(void *arg) */ static void tap_sock_tun_init(struct ctx *c) { + int i; + if (!c->splice_only) { NS_CALL(tap_ns_tun, c); if (c->fd_tap == -1) die("Failed to set up tap device in namespace"); }

+ /* Nothing else here runs if this fails: c->vhost.fd stays -1 and we + * carry on reading the tap fd ourselves, unless acceleration was + * explicitly asked for + */ + if (c->vhost.mode != VHOST_MODE_OFF && vhost_setup_net(c) < 0 && + c->vhost.mode == VHOST_MODE_ON) + die("vhost-net is not available"); + + if (c->vhost.fd != -1) { + for (i = 0; i < ARRAY_SIZE(c->vhost.vq); i++) + vhost_setup_eventfds(c, i); + + if (vhost_setup_memory_table(c) < 0) + die_perror("VHOST_SET_MEM_TABLE ioctl failed"); + + for (i = 0; i < ARRAY_SIZE(c->vhost.vq); i++) + vhost_set_vring(c, i, c->fd_tap); + } + pasta_ns_conf(c);

if (!c->splice_only) diff --git a/tap.h b/tap.h index f8c30e2..0f1f7a3 100644 --- a/tap.h +++ b/tap.h @@ -48,6 +48,8 @@ static inline void tap_hdr_update(struct tap_hdr *thdr, size_t l2len) thdr->vnet_len = htonl(l2len); }

+void tap_vhost_input(struct ctx *c, union epoll_ref ref, + const struct timespec *now); unsigned long tap_l2_max_len(const struct ctx *c); void *tap_push_l2h(const struct ctx *c, void *buf, const void *src_mac, uint16_t proto); -- 2.39.5 (Apple Git-154)