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([2a01:e0a:e10:ef90:343a:68f:2e91:95c]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-429dc20198csm5481863f8f.47.2025.11.04.08.28.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 04 Nov 2025 08:28:53 -0800 (PST) Message-ID: <7d5858a5-0e13-48cf-be9a-7cd7cb47202d@redhat.com> Date: Tue, 4 Nov 2025 17:28:52 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 5/5] tcp, udp: Pad batched frames for vhost-user modes to 60 bytes (802.3 minimum) To: Stefano Brivio References: <20251103101612.1412079-1-sbrivio@redhat.com> <20251103101612.1412079-6-sbrivio@redhat.com> <9d79c64e-e222-4444-8677-81891cd8fea3@redhat.com> <20251104170943.51dd7d04@elisabeth> 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: <20251104170943.51dd7d04@elisabeth> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: C6dYW48cGpghhLi4pnGm1Je8M5NS8XVrEMRpSn6361I_1762273734 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: PX3QKDWAYYGGKKKBX26U3IEEC67HWEDA X-Message-ID-Hash: PX3QKDWAYYGGKKKBX26U3IEEC67HWEDA 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, 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: On 11/4/25 17:09, Stefano Brivio wrote: > On Tue, 4 Nov 2025 16:50:43 +0100 > Laurent Vivier wrote: > >> On 11/3/25 11:16, Stefano Brivio wrote: >>> For both TCP and UDP, we request vhost-user buffers that are large >>> enough to reach ETH_ZLEN (60 bytes), so padding is just a matter of >>> increasing the appropriate iov_len and clearing bytes in the buffer >>> as needed. >>> >>> Link: https://bugs.passt.top/show_bug.cgi?id=166 >>> Signed-off-by: Stefano Brivio >>> --- >>> tcp.c | 2 -- >>> tcp_internal.h | 1 + >>> tcp_vu.c | 27 +++++++++++++++++++++++++++ >>> udp_vu.c | 11 ++++++++++- >>> 4 files changed, 38 insertions(+), 3 deletions(-) >>> >>> diff --git a/tcp.c b/tcp.c >>> index e91c0cf..039688d 100644 >>> --- a/tcp.c >>> +++ b/tcp.c >>> @@ -335,8 +335,6 @@ enum { >>> }; >>> #endif >>> >>> -/* MSS rounding: see SET_MSS() */ >>> -#define MSS_DEFAULT 536 >>> #define WINDOW_DEFAULT 14600 /* RFC 6928 */ >>> >>> #define ACK_INTERVAL 10 /* ms */ >>> diff --git a/tcp_internal.h b/tcp_internal.h >>> index 5f8fb35..d2295c9 100644 >>> --- a/tcp_internal.h >>> +++ b/tcp_internal.h >>> @@ -12,6 +12,7 @@ >>> #define BUF_DISCARD_SIZE (1 << 20) >>> #define DISCARD_IOV_NUM DIV_ROUND_UP(MAX_WINDOW, BUF_DISCARD_SIZE) >>> >>> +#define MSS_DEFAULT /* and minimum */ 536 /* as it comes from minimum MTU */ >>> #define MSS4 ROUND_DOWN(IP_MAX_MTU - \ >>> sizeof(struct tcphdr) - \ >>> sizeof(struct iphdr), \ >>> diff --git a/tcp_vu.c b/tcp_vu.c >>> index 1c81ce3..7239401 100644 >>> --- a/tcp_vu.c >>> +++ b/tcp_vu.c >>> @@ -60,6 +60,29 @@ static size_t tcp_vu_hdrlen(bool v6) >>> return hdrlen; >>> } >>> >>> +/** >>> + * tcp_vu_pad() - Pad 802.3 frame to minimum length (60 bytes) if needed >>> + * @iov: iovec array storing 802.3 frame with TCP segment inside >>> + * @cnt: Number of entries in @iov >>> + */ >>> +static void tcp_vu_pad(struct iovec *iov, size_t cnt) >>> +{ >>> + size_t l2len, pad; >>> + >>> + ASSERT(iov_size(iov, cnt) >= sizeof(struct virtio_net_hdr_mrg_rxbuf)); >>> + l2len = iov_size(iov, cnt) - sizeof(struct virtio_net_hdr_mrg_rxbuf); >>> + if (l2len >= ETH_ZLEN) >>> + return; >>> + >>> + pad = ETH_ZLEN - l2len; >>> + >>> + /* tcp_vu_sock_recv() requests at least MSS-sized vhost-user buffers */ >>> + static_assert(ETH_ZLEN <= MSS_DEFAULT); >>> + >>> + memset(&iov[cnt - 1].iov_base + iov[cnt - 1].iov_len, 0, pad); >> >> I think it should be >> >> memset((char *)iov[cnt - 1].iov_base + iov[cnt - 1].iov_len, 0, pad); > > Right, thanks, I always forget that sizeof(void) being 1 is a gcc > extension: > > https://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html > > What's rather confusing, actually, is that even if I explicitly enable > -Wpointer-arith, I don't get a warning for that. Any clue? in fact it's sizeof(void **) as iov[cnt - 1].iov_base is (void *) and you take &(void *). Normally gcc spits out warnings when we do .iov_base + .iov_len, but as you take address of iov_base all is fine :P Thanks, Laurent