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Fri, 26 Jun 2026 08:47:02 +0000 Date: Fri, 26 Jun 2026 09:46:58 +0100 From: Bruce Richardson To: Ciara Loftus CC: Subject: Re: [PATCH 0/3] net/intel: fix potential rx stats underflow Message-ID: References: <20260625093619.726471-1-ciara.loftus@intel.com> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20260625093619.726471-1-ciara.loftus@intel.com> X-ClientProxiedBy: DUZPR01CA0057.eurprd01.prod.exchangelabs.com (2603:10a6:10:469::6) To IA3PR11MB9421.namprd11.prod.outlook.com (2603:10b6:208:578::9) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA3PR11MB9421:EE_|SA2PR11MB5051:EE_ X-MS-Office365-Filtering-Correlation-Id: 3b5278bd-cb62-4028-2624-08ded35f7e04 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; ARA:13230040|376014|366016|23010399003|1800799024|18002099003|22082099003|11063799006|56012099006|5023799004; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:255.255.255.255; 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That sum already includes the > discarded packets, so the result is the number actually delivered. The > inputs are sampled from separate sources at slightly different instants, > so the discard count can briefly exceed the measured sum. Because the > arithmetic is unsigned, the subtraction wraps to a huge value and is > reported as a hugely incorrect packet count and rate. > > All three share the bug but not the fix, because they differ in how > much control they have over the sampling. > > The ice PF reads its counters directly from hardware registers, so the > order is under the driver's control. Reading the discard register before > the other three, combined with the fact that the counters only ever > increase and the delivered-packet sum always includes the discards, > makes it impossible for the discard count to exceed the later sum. No > clamping is needed and the subtraction can never underflow. > > ice DCF and iavf receive their counters from the PF in a single virtchnl > message and cannot influence the order in which the PF sampled them. > There a reorder is not available, so the subtraction is made saturating: > when the discard count exceeds the sum, essentially nothing was delivered, > so zero is reported instead of underflowing. > > Ciara Loftus (3): > net/ice: fix Rx packets statistics underflow > net/ice: fix DCF Rx packets statistics underflow > net/iavf: fix Rx packets statistics underflow > Series applied to dpdk-next-net-intel. I was tempted to squash into one patch, but kept them as three because the fix is not identical in all cases (iavf being different). Thanks, /Bruce