From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: passt.top; dmarc=none (p=none dis=none) header.from=gibson.dropbear.id.au Authentication-Results: passt.top; dkim=pass (2048-bit key; secure) header.d=gibson.dropbear.id.au header.i=@gibson.dropbear.id.au header.a=rsa-sha256 header.s=202502 header.b=HvBJWJzp; dkim-atps=neutral Received: from mail.ozlabs.org (mail.ozlabs.org [IPv6:2404:9400:2221:ea00::3]) by passt.top (Postfix) with ESMTPS id 11D1B5A061B for ; Thu, 13 Mar 2025 06:41:02 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gibson.dropbear.id.au; s=202502; t=1741844452; bh=CiR1OjXiYmfpTv5/JyzoFhpM4/O+0ygHrTGCw1lHj+0=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=HvBJWJzp8ORP4cLYP+Fz9cXVK/JhZQ7c9OFfhYoSViCk14n3vAUE61gNTZ+0BgJkl L67Guev4298S1J3T2hwudz1bgkcbuEXB+dhMZ2udhEDKVU4DRFMSRtS3VRTufmu3fQ YvNyKqAKvgtuVbmOj6eoRpTdLz8j0dPcd5b7XTrK3ZmivQ31RIRuVBeF5VOgFigrtg 8l+EieDHP95aFwNaIMz2hNuqownOHTvalB0T/KcNo7/FO4coC7dKLfAhDXqSgJqNUV 1E6ZVyDPv40vepjsVk+Khrxq1WvwS662BqBvQVWr3+80IvPQAPK5zWZsfEA/lSWrC4 nOoWCthaYnZcQ== Received: by gandalf.ozlabs.org (Postfix, from userid 1007) id 4ZCxDh4nBSz4x89; Thu, 13 Mar 2025 16:40:52 +1100 (AEDT) From: David Gibson To: passt-dev@passt.top, Stefano Brivio Subject: [PATCH 4/4] tap: Clarify calculation of TAP_MSGS Date: Thu, 13 Mar 2025 16:40:50 +1100 Message-ID: <20250313054050.642978-5-david@gibson.dropbear.id.au> X-Mailer: git-send-email 2.48.1 In-Reply-To: <20250313054050.642978-1-david@gibson.dropbear.id.au> References: <20250313054050.642978-1-david@gibson.dropbear.id.au> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Message-ID-Hash: GSASIONJ7HH7WVUUGP7EQKXI7J4VGJJA X-Message-ID-Hash: GSASIONJ7HH7WVUUGP7EQKXI7J4VGJJA X-MailFrom: dgibson@gandalf.ozlabs.org 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: David Gibson 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: 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 --- tap.c | 28 ++++++++++++++++++++++------ 1 file changed, 22 insertions(+), 6 deletions(-) diff --git a/tap.c b/tap.c index 34e6774f..3a6fcbe8 100644 --- a/tap.c +++ b/tap.c @@ -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); -- 2.48.1