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Message-ID: Date: Wed, 14 Aug 2024 11:55:10 +0100 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 2/4] xen: make VMAP only support in MMU system Content-Language: en-GB To: Jan Beulich , Ayan Kumar Halder Cc: xen-devel@lists.xenproject.org, Penny Zheng , Wei Chen , sstabellini@kernel.org, bertrand.marquis@arm.com, michal.orzel@amd.com, Volodymyr_Babchuk@epam.com, julien@xen.org References: <20240813171356.46760-1-ayan.kumar.halder@amd.com> <20240813171356.46760-3-ayan.kumar.halder@amd.com> <6d333d94-80ad-485c-b024-a45a388b96a4@suse.com> From: Ayan Kumar Halder In-Reply-To: <6d333d94-80ad-485c-b024-a45a388b96a4@suse.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: FR4P281CA0301.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:f6::13) To PH8PR12MB7326.namprd12.prod.outlook.com (2603:10b6:510:216::7) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH8PR12MB7326:EE_|CY5PR12MB6624:EE_ X-MS-Office365-Filtering-Correlation-Id: de5e0c70-47e7-4cdb-1690-08dcbc4f94e5 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014; 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Since this is highly dependent on virtual >> address translation, we choose to fold VMAP in MMU system. >> >> In this patch, we introduce a new Kconfig CONFIG_HAS_VMAP, and make it >> only support in MMU system on ARM architecture. And ALTERNATIVE now >> depends on VMAP. >> >> HARDEN_BRANCH_PREDICTOR is now gated on HAS_VMAP as speculative >> attacks are not possible on non MMU based systems (ie Cortex-R52, R82). >> See https://developer.arm.com/Arm%20Security%20Center/Speculative%20Processor%20Vulnerability. > While I'm not an Arm expert and hence I'm likely missing aspects, I question > the one (Spectre-BHB) vulnerability there to be sufficient to draw a > conclusion towards the usefulness of branch hardening. I would advise > against encoding such a connection in the Kconfig dependencies. AFAIU, to address 'Spectre' like vulnerabilities 'branch hardening' was added. See https://lore.kernel.org/all/E1fNadD-0000fz-9r@rmk-PC.armlinux.org.uk/ And from https://lists.linaro.org/archives/list/linux-stable-mirror@lists.linaro.org/message/F4MGL4WT2R7T54NLRDGKFRQNSXF3DZGD/ Spectre is valid on MMU based systems. Thus, I would make 'branch hardenining' valid on MMU based systems only. Let me know your thoughts. > >> --- a/xen/arch/x86/Kconfig >> +++ b/xen/arch/x86/Kconfig >> @@ -1,6 +1,7 @@ >> config X86_64 >> def_bool y >> select 64BIT >> + select HAS_VMAP >> >> config X86 >> def_bool y >> @@ -31,6 +32,7 @@ config X86 >> select HAS_UBSAN >> select HAS_VPCI if HVM >> select NEEDS_LIBELF >> + select HAS_VMAP > Why in two places? Also please respect alphabetic sorting here (if this > hunk is kept, which may be the more consistent approach) ... My mistake. I initially thought user could select one of the two. However, "vm_init()" is invoked only from xen/arch/x86/setup.c. Thus, keeping 'HAS_VMAP' under 'config X86' is dufficient. I agree that HAS_VMAP needs to go before HAS_VPCI. > >> --- a/xen/common/Kconfig >> +++ b/xen/common/Kconfig >> @@ -77,6 +77,9 @@ config HAS_PIRQ >> config HAS_PMAP >> bool >> >> +config HAS_VMAP >> + bool >> + >> config HAS_SCHED_GRANULARITY >> bool > ... and here. Yes, it needs to go after HAS_UBSAN. - Ayan > > Jan