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Wed, 28 Oct 2026 10:25:33 -0700 (PDT) Received: from localhost ([153.61.198.248]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3ae6e9583c1sm397200a91.10.2026.10.28.10.25.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 28 Oct 2026 10:25:32 -0700 (PDT) Content-Type: text/plain; charset=UTF-8 Date: Wed, 28 Oct 2026 17:25:31 +0000 Message-Id: Subject: Re: [PATCH RFC 0/8] vsock: add datagram support to virtio and vhost From: "Alexei Starovoitov" To: "Sergio Lopez" , "Stefano Garzarella" , "Bobby Eshleman" , "David S. Miller" , "Eric Dumazet" , "Jakub Kicinski" , "Paolo Abeni" , "Simon Horman" , "Bryan Tan" , "Vishnu Dasa" , "Broadcom internal kernel review list" , "Stefan Hajnoczi" , "Michael S. Tsirkin" , "Jason Wang" , =?utf-8?q?Eugenio_P=C3=A9rez?= , "Xuan Zhuo" , "Haiyang Zhang" , "Wei Liu" , "Dexuan Cui" Cc: , , , , , In-Reply-To: <20261028-vsock-dgram-support-v1-0-e457cf7cee78@sinrega.org> References: <20261028-vsock-dgram-support-v1-0-e457cf7cee78@sinrega.org> Content-Transfer-Encoding: 8bit X-Mailing-List: kvm@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 On Wed, Oct 28, 2026 at 09:04 AM Sergio Lopez wrote: > This series implements SOCK_DGRAM support for virtio-vsock and > vhost-vsock, allowing applications to exchange connectionless messages > through the existing AF_VSOCK datagram API. The cover doesn't say who needs it. Bobby and Amery took the same feature to v6: https://lore.kernel.org/all/20240710212555.1617795-1-amery.hung@bytedance.com/ Stefano said there that it can work well and that the virtio spec change is the next step. The last mails of the thread have Amery's v7 WIP and a short list of what is left. The cover doesn't mention it. The spec change is still missing. Why not finish v7 instead ? This RFC goes the other way in these places: what v6 this RFC ----------------- ------------------------ ---------------------------- (cid, port) space own bind table for dgram shared with stream. The 2nd bind() gets EADDRINUSE vsk->transport empty until connect(), g2h or h2g, picked in lookup per packet socket(). -ENODEV with vsock_loopback only routing one helper for stream 2nd copy of the chain in and dgram vsock_assign_transport() recvmsg() in af_vsock.c, dgram_dequeue() stays, no dgram_dequeue() dgram_payload_len() is new VMCI fallback first. No virtio dgram when its module is loaded dgram > RX buffer drop, EHOSTUNREACH in every RX buffer is 64K the error queue > Datagrams use destination-based routing across guest-to-host, > host-to-guest and loopback transports while retaining the socket's > transport state. VMCI keeps priority as the dedicated datagram A dgram socket uses two members of struct virtio_vsock_sock: tx_lock and bytes_unsent. They serve SIOCOUTQ only. virtio_transport_alloc_skb() does skb_set_owner_w() already. udp_ioctl() returns sk_wmem_alloc_get() for SIOCOUTQ. Without them a dgram socket has no transport state, like in v6. Then no need for the owner transport, dgram_transport_match(), fallback_transport, tx_accounted, the skb destructor, etc. > transport. Guest RX buffers accommodate complete messages when > datagrams are negotiated, preserving message boundaries. That's 64K per RX buffer instead of 4K, for stream and seqpacket too. It undoes commit 03a92f036a04 ("vsock/virtio: Resize receive buffers so that each SKB fits in a 4K page"). pskb_trim_unique() doesn't reduce skb->truesize. So a datagram of 1 byte charges more than 64K to sk_rmem_alloc of the guest socket. The default sk_rcvbuf is about 208K on x86-64. The 5th datagram in the queue is dropped.