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b=MWysoe1HRYBASFKmqsbsya7o61qbNeFZxPpjRUr+O5ce1sA96lHn/W47uuzqteJC54ekwG4GO8S/F1deDz0+jO1IaJE7Oxi52Zr0hRv1DBxKMoC0VaAItgpskcynfFdJqeujbajz/McumUhzPo+mcc7CXH1o1OLnflrYmlTnXDI= Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=amd.com; Received: from BL3PR12MB9049.namprd12.prod.outlook.com (2603:10b6:208:3b8::21) by DM6PR12MB4353.namprd12.prod.outlook.com (2603:10b6:5:2a6::12) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.181.13; Mon, 6 Jul 2026 20:11:27 +0000 Received: from BL3PR12MB9049.namprd12.prod.outlook.com ([fe80::ae6a:9bdd:af5b:e9ad]) by BL3PR12MB9049.namprd12.prod.outlook.com ([fe80::ae6a:9bdd:af5b:e9ad%7]) with mapi id 15.21.0181.012; Mon, 6 Jul 2026 20:11:27 +0000 Message-ID: <151e544a-63dc-414f-8418-e6912d196a9e@amd.com> Date: Mon, 6 Jul 2026 15:11:25 -0500 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v10 3/6] x86/sev: Disable CPU hotplug while SNP is active To: sashiko-reviews@lists.linux.dev Cc: kvm@vger.kernel.org References: <205a5259f9fd353dc0ca6b00565c8175a96768c7.1782841284.git.ashish.kalra@amd.com> <20260630183231.393071F000E9@smtp.kernel.org> Content-Language: en-US From: "Kalra, Ashish" Autocrypt: addr=ashish.kalra@amd.com; 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Sashiko AI review found 1 potential issue(s) to consider: > - [Critical] System suspend (PM transitions) bypasses `cpu_hotplug_disable()`, stripping `SnpEn` from secondary CPUs on resume. > -- > > commit 29b8362e9ad72efa8892f6bb3124dbbcc29ee822 > Author: Ashish Kalra > > x86/sev: Disable CPU hotplug while SNP is active > > This commit disables CPU hotplug in snp_prepare() before SNP is enabled and > re-enables it in snp_shutdown(). This aims to prevent CPUs from coming online > without the SnpEn bit set while SNP is active, as the firmware cannot track > OS CPU hotplug events. > >> diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c >> index dab6e1c290bca..04a58ac4339cb 100644 >> --- a/arch/x86/virt/svm/sev.c >> +++ b/arch/x86/virt/svm/sev.c >> @@ -535,6 +535,15 @@ int snp_prepare(void) >> >> clear_rmp(); >> >> + /* >> + * Disable CPU hotplug before enabling SNP, so no CPU can come online >> + * without SnpEn while SNP is enabled; it is re-enabled in snp_shutdown() >> + * once SNP is disabled. Must be before cpus_read_lock(): >> + * cpu_hotplug_disable() takes cpu_add_remove_lock, which nests above >> + * cpu_hotplug_lock. >> + */ >> + cpu_hotplug_disable(); > > [Severity: Critical] > Does this actually prevent all CPUs from coming online without SnpEn? > > Looking at system suspend (PM transitions), freeze_secondary_cpus() calls > _cpu_down() directly, which bypasses the cpu_hotplug_disabled check. Upon > resume, thaw_secondary_cpus() brings them back online via _cpu_up(). > > Since snp_enable() is called via a one-off on_each_cpu() in snp_prepare() > rather than being registered as a CPU hotplug callback, wouldn't secondary > CPUs resume with the SnpEn MSR bit cleared? > > This could allow a suspend/resume cycle to create an asymmetrical state where > hardware RMP checks are bypassed. > > [ ... ] > You're right about the mechanism — cpu_hotplug_disable() doesn't cover the suspend freeze/thaw path, and snp_enable() has no resume hook. But an SNP host doesn't support suspend/resume: SNP is set up once at boot (SnpEn on all CPUs followed by SNP_INIT), and that state can't be rebuilt on resume without a full re-init. So this can't arise in practice, and it's pre-existing base-SNP behavior, not something this patch introduces. Thanks, Ashish