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Message-ID: <7da3506c-dc2a-aa50-2bac-2985d2b9e66e@suse.com> Date: Thu, 11 Aug 2022 12:59:33 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.12.0 Subject: Re: [PATCH] x86/cpu: Drop _init from *_cpu_cap functions Content-Language: en-US To: Andrew Cooper , Ross Lagerwall Cc: Roger Pau Monne , Wei Liu , "xen-devel@lists.xenproject.org" References: <20220811101715.3947873-1-ross.lagerwall@citrix.com> <7ff6d006-0a6d-60e9-ebb7-70ad3c6d6a5b@citrix.com> From: Jan Beulich In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: FR0P281CA0041.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:48::6) To VE1PR04MB6560.eurprd04.prod.outlook.com (2603:10a6:803:122::25) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 309941d8-3b4b-4e00-ba8b-08da7b8893e3 X-MS-TrafficTypeDiagnostic: AM6PR04MB6744:EE_ X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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xen-devel@lists.xenproject.org >>> Cc: Jan Beulich ; Roger Pau Monne ; Wei Liu >>> Subject: Re: [PATCH] x86/cpu: Drop _init from *_cpu_cap functions >>>   >>> On 11/08/2022 11:17, Ross Lagerwall wrote: >>>> These functions may be called by init_amd() after the _init functions >>>> have been purged during CPU hotplug or PV shim boot so drop the _init. >>>> >>>> Signed-off-by: Ross Lagerwall >>> Hmm.  That's a bug in init_amd() I'd say.  These really shouldn't be >>> used after __init. >>> >>> Which path exploded specifically? >> The stack trace was: >> >> setup_force_cpu_cap >> init_amd >> identify_cpu >> start_secondary >> >> In setup_force_cpu_cap() here: >> >> /* >> * On pre-CLFLUSHOPT AMD CPUs, CLFLUSH is weakly ordered with >> * everything, including reads and writes to address, and >> * LFENCE/SFENCE instructions. >> */ >> if (!cpu_has_clflushopt) >> setup_force_cpu_cap(X86_BUG_CLFLUSH_MFENCE); >> >> which was recently introduced by: >> >> commit 062868a5a8b428b85db589fa9a6d6e43969ffeb9 >> Author: Andrew Cooper >> Date: Thu Jun 9 14:23:07 2022 +0200 >> >> x86/amd: Work around CLFLUSH ordering on older parts > > Bah, and that was also backported in a security fix, to everything back > to 4.12 is broken. 4.13, but yes. Oh well. It's actually odd that we use __set_bit() for X86_FEATURE_MFENCE_RDTSC (a few lines up) but the more heavyweight function for X86_BUG_CLFLUSH_MFENCE. Jan