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Peter Anvin" , "Lai, Yi" , Xin Li References: <43f2c5c2-6327-4546-b204-5c66b704042e@citrix.com> <568aa6c4-6802-4eb5-b412-e3aa93ed9b29@intel.com> <91590BC4-A54E-40F4-A223-A07FBA386A9A@zytor.com> Content-Language: en-GB From: Andrew Cooper In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: PA7P264CA0112.FRAP264.PROD.OUTLOOK.COM (2603:10a6:102:34c::15) To CH8PR03MB8275.namprd03.prod.outlook.com (2603:10b6:610:2b9::7) Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CH8PR03MB8275:EE_|DS7PR03MB5510:EE_ X-MS-Office365-Filtering-Correlation-Id: 5d118a58-6a7b-4883-c9f6-08de8910dcce X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014|7416014|22082099003|56012099003|18002099003; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:CH8PR03MB8275.namprd03.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230040)(366016)(1800799024)(376014)(7416014)(22082099003)(56012099003)(18002099003);DIR:OUT;SFP:1101; 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Peter Anvin wrote: > On 2026-03-22 23:06, Lai, Yi wrote: >> Thanks for the context and links, Xin. >> >> My patch is a simple, focused fix to unblock current regression in >> 'sysret_rip'. The goal is just to ensure it doesn't falsely fail on FRED >> systems by skipping the IDT assertion (R11==RFLAGS). >> >> Checking Ammar's patch series, it indeed looks like a more comprehensive >> solution that not only handles the difference but also adds specific >> test coverage for consistency. Is there a specific reason it didn't >> land in the mainline kernel? If you think that is the preferred >> direction, I would be happy to contribute to that effort >> collaboratively. >> > So for what it's worth, I have used this sequence to probe for FRED, but it is > basically probing for the effect you are truing to test for here: > > #include > #include > #include > > bool fred_is_on(void) > { > const uint64_t sentinel1 = 0xfeedfacedeadbeef; > const uint64_t sentinel2 = 0xbeeffeeb1ef00d1e; > > register uint64_t rax asm("rax") = __NR_getppid; > register uint64_t rcx asm("rcx") = sentinel1; > register uint64_t r11 asm("r11") = sentinel2; > > asm volatile("syscall" : "+r" (rax), "+r" (rcx), "+r" (r11)); > > return !((rcx ^ sentinel1) | (r11 ^ sentinel2)); > } > > -hpa This doesn't really test whether FRED is active.  It tests whether the OS is not providing strict backwards compatibility, and I think will malfunction when there's a hypervisor above Linux providing strict backwards compatibility. ~Andrew