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([2a01:e0a:e10:ef90:343a:68f:2e91:95c]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-6507bf667fcsm6578725a12.29.2026.01.07.23.13.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 07 Jan 2026 23:13:03 -0800 (PST) Message-ID: Date: Thu, 8 Jan 2026 08:13:03 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] udp_vu: Discard datagrams when RX virtqueue is not usable To: David Gibson References: <20260107080809.3440007-1-lvivier@redhat.com> From: Laurent Vivier Autocrypt: addr=lvivier@redhat.com; keydata= xsFNBFYFJhkBEAC2me7w2+RizYOKZM+vZCx69GTewOwqzHrrHSG07MUAxJ6AY29/+HYf6EY2 WoeuLWDmXE7A3oJoIsRecD6BXHTb0OYS20lS608anr3B0xn5g0BX7es9Mw+hV/pL+63EOCVm SUVTEQwbGQN62guOKnJJJfphbbv82glIC/Ei4Ky8BwZkUuXd7d5NFJKC9/GDrbWdj75cDNQx UZ9XXbXEKY9MHX83Uy7JFoiFDMOVHn55HnncflUncO0zDzY7CxFeQFwYRbsCXOUL9yBtqLer Ky8/yjBskIlNrp0uQSt9LMoMsdSjYLYhvk1StsNPg74+s4u0Q6z45+l8RAsgLw5OLtTa+ePM JyS7OIGNYxAX6eZk1+91a6tnqfyPcMbduxyBaYXn94HUG162BeuyBkbNoIDkB7pCByed1A7q q9/FbuTDwgVGVLYthYSfTtN0Y60OgNkWCMtFwKxRaXt1WFA5ceqinN/XkgA+vf2Ch72zBkJL RBIhfOPFv5f2Hkkj0MvsUXpOWaOjatiu0fpPo6Hw14UEpywke1zN4NKubApQOlNKZZC4hu6/ 8pv2t4HRi7s0K88jQYBRPObjrN5+owtI51xMaYzvPitHQ2053LmgsOdN9EKOqZeHAYG2SmRW LOxYWKX14YkZI5j/TXfKlTpwSMvXho+efN4kgFvFmP6WT+tPnwARAQABzSNMYXVyZW50IFZp dmllciA8bHZpdmllckByZWRoYXQuY29tPsLBeAQTAQIAIgUCVgVQgAIbAwYLCQgHAwIGFQgC CQoLBBYCAwECHgECF4AACgkQ8ww4vT8vvjwpgg//fSGy0Rs/t8cPFuzoY1cex4limJQfReLr SJXCANg9NOWy/bFK5wunj+h/RCFxIFhZcyXveurkBwYikDPUrBoBRoOJY/BHK0iZo7/WQkur 6H5losVZtrotmKOGnP/lJYZ3H6OWvXzdz8LL5hb3TvGOP68K8Bn8UsIaZJoeiKhaNR0sOJyI YYbgFQPWMHfVwHD/U+/gqRhD7apVysxv5by/pKDln1I5v0cRRH6hd8M8oXgKhF2+rAOL7gvh jEHSSWKUlMjC7YwwjSZmUkL+TQyE18e2XBk85X8Da3FznrLiHZFHQ/NzETYxRjnOzD7/kOVy gKD/o7asyWQVU65mh/ECrtjfhtCBSYmIIVkopoLaVJ/kEbVJQegT2P6NgERC/31kmTF69vn8 uQyW11Hk8tyubicByL3/XVBrq4jZdJW3cePNJbTNaT0d/bjMg5zCWHbMErUib2Nellnbg6bc 2HLDe0NLVPuRZhHUHM9hO/JNnHfvgiRQDh6loNOUnm9Iw2YiVgZNnT4soUehMZ7au8PwSl4I KYE4ulJ8RRiydN7fES3IZWmOPlyskp1QMQBD/w16o+lEtY6HSFEzsK3o0vuBRBVp2WKnssVH qeeV01ZHw0bvWKjxVNOksP98eJfWLfV9l9e7s6TaAeySKRRubtJ+21PRuYAxKsaueBfUE7ZT 7zfOwU0EVgUmGQEQALxSQRbl/QOnmssVDxWhHM5TGxl7oLNJms2zmBpcmlrIsn8nNz0rRyxT 460k2niaTwowSRK8KWVDeAW6ZAaWiYjLlTunoKwvF8vP3JyWpBz0diTxL5o+xpvy/Q6YU3BN efdq8Vy3rFsxgW7mMSrI/CxJ667y8ot5DVugeS2NyHfmZlPGE0Nsy7hlebS4liisXOrN3jFz asKyUws3VXek4V65lHwB23BVzsnFMn/bw/rPliqXGcwl8CoJu8dSyrCcd1Ibs0/Inq9S9+t0 VmWiQWfQkz4rvEeTQkp/VfgZ6z98JRW7S6l6eophoWs0/ZyRfOm+QVSqRfFZdxdP2PlGeIFM C3fXJgygXJkFPyWkVElr76JTbtSHsGWbt6xUlYHKXWo+xf9WgtLeby3cfSkEchACrxDrQpj+ Jt/JFP+q997dybkyZ5IoHWuPkn7uZGBrKIHmBunTco1+cKSuRiSCYpBIXZMHCzPgVDjk4viP brV9NwRkmaOxVvye0vctJeWvJ6KA7NoAURplIGCqkCRwg0MmLrfoZnK/gRqVJ/f6adhU1oo6 z4p2/z3PemA0C0ANatgHgBb90cd16AUxpdEQmOCmdNnNJF/3Zt3inzF+NFzHoM5Vwq6rc1JP jfC3oqRLJzqAEHBDjQFlqNR3IFCIAo4SYQRBdAHBCzkM4rWyRhuVABEBAAHCwV8EGAECAAkF AlYFJhkCGwwACgkQ8ww4vT8vvjwg9w//VQrcnVg3TsjEybxDEUBm8dBmnKqcnTBFmxN5FFtI WlEuY8+YMiWRykd8Ln9RJ/98/ghABHz9TN8TRo2b6WimV64FmlVn17Ri6FgFU3xNt9TTEChq AcNg88eYryKsYpFwegGpwUlaUaaGh1m9OrTzcQy+klVfZWaVJ9Nw0keoGRGb8j4XjVpL8+2x OhXKrM1fzzb8JtAuSbuzZSQPDwQEI5CKKxp7zf76J21YeRrEW4WDznPyVcDTa+tz++q2S/Bp P4W98bXCBIuQgs2m+OflERv5c3Ojldp04/S4NEjXEYRWdiCxN7ca5iPml5gLtuvhJMSy36gl U6IW9kn30IWuSoBpTkgV7rLUEhh9Ms82VWW/h2TxL8enfx40PrfbDtWwqRID3WY8jLrjKfTd R3LW8BnUDNkG+c4FzvvGUs8AvuqxxyHbXAfDx9o/jXfPHVRmJVhSmd+hC3mcQ+4iX5bBPBPM oDqSoLt5w9GoQQ6gDVP2ZjTWqwSRMLzNr37rJjZ1pt0DCMMTbiYIUcrhX8eveCJtY7NGWNyx FCRkhxRuGcpwPmRVDwOl39MB3iTsRighiMnijkbLXiKoJ5CDVvX5yicNqYJPKh5MFXN1bvsB kmYiStMRbrD0HoY1kx5/VozBtc70OU0EB8Wrv9hZD+Ofp0T3KOr1RUHvCZoLURfFhSQ= In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: LQyalqyaLTwtaYwY5Amsf22laFjKArr9d4nnZAmQEn4_1767856385 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Message-ID-Hash: 55WUL47M43C7SRTKOHEQZILDHEZBM7L7 X-Message-ID-Hash: 55WUL47M43C7SRTKOHEQZILDHEZBM7L7 X-MailFrom: lvivier@redhat.com X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: passt-dev@passt.top X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: On 1/8/26 00:48, David Gibson wrote: > On Wed, Jan 07, 2026 at 09:08:09AM +0100, Laurent Vivier wrote: >> During vhost-user device initialization, UDP datagrams may arrive on >> listening sockets before the guest has enabled the RX virtqueue. >> >> When this happens, udp_vu_sock_recv() returns 0 without consuming >> the datagram from the socket. The caller, udp_sock_fwd(), uses a >> while loop with udp_peek_addr() to process pending datagrams. Since >> the datagram remains in the socket buffer, udp_peek_addr() keeps >> returning data available, causing a busy loop with 100% CPU usage. >> >> To avoid that, we need to discard the data when the virtqueue is not >> ready. udp_buf_sock_to_tap() actually does the same as it reads data >> with udp_sock_recv() and if fd_tap is not initialized tap_send_frames() >> drops them. >> >> Fixes: 28997fcb29b5 ("vhost-user: add vhost-user") >> Link: https://bugs.passt.top/show_bug.cgi?id=185 >> Signed-off-by: Laurent Vivier > > Reviewed-by: David Gibson > > Although one possible nit noted.. > >> --- >> >> Notes: >> v2: >> - move recvmsg() from udp_vu_sock_to_tap() to udp_vu_sock_recv() >> >> udp_vu.c | 32 ++++++++++++++++++++++---------- >> 1 file changed, 22 insertions(+), 10 deletions(-) >> >> diff --git a/udp_vu.c b/udp_vu.c >> index c30dcf97698f..3774d538a2d0 100644 >> --- a/udp_vu.c >> +++ b/udp_vu.c >> @@ -65,7 +65,8 @@ static size_t udp_vu_hdrlen(bool v6) >> * @v6: Set for IPv6 connections >> * @dlen: Size of received data (output) >> * >> - * Return: number of iov entries used to store the datagram >> + * Return: number of iov entries used to store the datagram, 0 if the datagram >> + * was discarded because the virtqueue is not ready, -1 on error >> */ >> static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s, >> bool v6, ssize_t *dlen) >> @@ -77,6 +78,15 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s, >> >> ASSERT(!c->no_udp); >> >> + if (!vu_queue_enabled(vq) || !vu_queue_started(vq)) { >> + debug("Got UDP packet, but RX virtqueue not usable yet"); >> + >> + if (recvmsg(s, &msg, MSG_DONTWAIT) < 0) > > You use MSG_DONTWAIT here, but you don't on the normal path. I guess > it shouldn't make a difference, since we've come from epoll so we know > something is waiting for us. But I think we want to make the paths > look as identical as we can from the point of view of the socket side, > and this makes it a bit less obvious. This is actually consistent with the existing discard pattern in udp_sock_fwd(). When udp_sock_fwd() needs to discard a datagram, it does exactly the same thing: if (discard) { struct msghdr msg = { 0 }; if (recvmsg(s, &msg, MSG_DONTWAIT) < 0) debug_perror("Failed to discard datagram"); } udp_sock_fwd() loops on udp_peek_addr(), calls udp_vu_sock_to_tap() (and then udp_vu_sock_recv()). If something has to be discarded it calls recvmsg(s, &msg, MSG_DONTWAIT). Thanks, Laurent > >> + debug_perror("Failed to discard datagram"); > > This also isn't really accurate in the case of EAGAIN / EWOULDBLOCK. > >> + >> + return 0; >> + } >> + >> /* compute L2 header length */ >> hdrlen = udp_vu_hdrlen(v6); >> >> @@ -87,7 +97,7 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s, >> sizeof(struct virtio_net_hdr_mrg_rxbuf), >> NULL); >> if (iov_cnt == 0) >> - return 0; >> + return -1; >> >> /* reserve space for the headers */ >> ASSERT(iov_vu[0].iov_len >= MAX(hdrlen, ETH_ZLEN)); >> @@ -101,7 +111,7 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s, >> *dlen = recvmsg(s, &msg, 0); >> if (*dlen < 0) { >> vu_queue_rewind(vq, iov_cnt); >> - return 0; >> + return -1; >> } >> >> /* restore the pointer to the headers address */ >> @@ -216,15 +226,17 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx) >> int iov_used; >> >> iov_used = udp_vu_sock_recv(c, vq, s, v6, &dlen); >> - if (iov_used <= 0) >> + if (iov_used < 0) >> break; >> >> - udp_vu_prepare(c, toside, dlen); >> - if (*c->pcap) { >> - udp_vu_csum(toside, iov_used); >> - pcap_iov(iov_vu, iov_used, >> - sizeof(struct virtio_net_hdr_mrg_rxbuf)); >> + if (iov_used > 0) { >> + udp_vu_prepare(c, toside, dlen); >> + if (*c->pcap) { >> + udp_vu_csum(toside, iov_used); >> + pcap_iov(iov_vu, iov_used, >> + sizeof(struct virtio_net_hdr_mrg_rxbuf)); >> + } >> + vu_flush(vdev, vq, elem, iov_used); >> } >> - vu_flush(vdev, vq, elem, iov_used); >> } >> } >> -- >> 2.52.0 >> >