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Message-ID: <2f5ec31e-2ac5-2b22-8801-9ebdc4946962@suse.com> Date: Thu, 20 Oct 2022 13:22:20 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.3.3 Subject: Re: Intended behavior/usage of SSBD setting Content-Language: en-US To: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Cc: Andrew Cooper , xen-devel@lists.xenproject.org References: From: Jan Beulich In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: FR3P281CA0145.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:95::8) To VE1PR04MB6560.eurprd04.prod.outlook.com (2603:10a6:803:122::25) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: VE1PR04MB6560:EE_|DB8PR04MB7020:EE_ X-MS-Office365-Filtering-Correlation-Id: e3da713e-cce5-459f-10c7-08dab28d5b75 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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:VE1PR04MB6560.eurprd04.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230022)(366004)(396003)(346002)(136003)(376002)(39860400002)(451199015)(36756003)(83380400001)(2616005)(31696002)(86362001)(186003)(38100700002)(8936002)(31686004)(66899015)(6916009)(4326008)(5660300002)(66476007)(66946007)(316002)(41300700001)(478600001)(66556008)(8676002)(6512007)(26005)(6506007)(6486002)(53546011)(2906002)(45980500001)(43740500002);DIR:OUT;SFP:1101; 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There's currently a `spec-ctrl=ssbd` option [0], > that has an out of date description, as now SSBD is always offered to > guests on AMD hardware, either using SPEC_CTRL or VIRT_SPEC_CTRL. > > It has been pointed out by Andrew that toggling SSBD on AMD using > VIRT_SPEC_CTRL or the non-architectural way (MSR_AMD64_LS_CFG) can > have a high impact on performance, and hence switching it on every > guest <-> hypervisor context switch is likely a very high > performance penalty. > > It's been suggested that it could be more appropriate to run Xen with > the guest SSBD selection on those systems, however that clashes with > the current intent of the `spec-ctrl=ssbd` option. > > I hope I have captured the expressed opinions correctly in the text > above. > > I see two ways to solve this: > > * Keep the current logic for switching SSBD on guest <-> hypervisor > context switch, but only use it if `spec-ctrl=ssbd` is set on the > command line. > > * Remove the logic for switching SSBD on guest <-> hypervisor context > switch, ignore setting of `spec-ctrl=ssbd` on those systems and run > hypervisor code with the guest selection of SSBD. * Give the guest the illusion of controlling the behavior, but run with SSBD always enabled when "spec-ctrl=ssbd" is in effect. * Give the guest the illusion of controlling the behavior when "spec-ctrl=ssbd" is in effect, running with the OR of guest and host settings (switched, if necessary, as vCPU-s are context-switched). Jan