Re: [PATCH v10 3/6] x86/sev: Disable CPU hotplug while SNP is active

["Kalra, Ashish" <ashish.kalra@amd.com>]

On 7/1/2026 4:40 AM, Jethro Beekman wrote:
> Hi Ashish,
> 
> I don't believe my concern has been addressed
> 
> https://lore.kernel.org/lkml/0df3b665-3a9c-4c46-a7aa-14388e8e1577@fortanix.com/
> 
> --

The disable tracks SNP_INIT, not "SNP" in general: SNP_INIT requires SnpEn to be set on all present CPUs, and a CPU brought online afterward wouldn't have it, so the kernel that runs SNP_INIT must keep its CPU set stable. Today the only kernel that runs SNP_INIT is the bare-metal host, so a plain L1 guest keeps full CPU hotplug.

Concretely, the path is gated by CC_ATTR_HOST_SEV_SNP, which bsp_determine_snp() sets only when X86_FEATURE_HYPERVISOR is clear and clears otherwise 
(as Prateek pointed out). So a Linux L1 guest never has it set, never reaches snp_prepare()/snp_rmptable_init(), and keeps CPU hotplug — 
including while running SEV/SEV-ES confidential L2 guests. Only SNP initialization disables hotplug; the other SEV variants don't. And KVM doesn't expose
SNP to L1, so an L1 can't be an SNP host today in any case.
  
On the nested scenario you raised: if SNP-guest-as-L2 support is added, an L1 acting as an SNP host would run a *virtualized* SNP_INIT. A faithful virtualization carries the same constraint as physical SNP_INIT — all present (v)CPUs must be SnpEn — so that L1 would have the same (v)CPU-hotplug-disable requirement, just over its virtual CPUs, and this same code would apply at that level. So the disable isn't too broad; it correctly tracks SNP_INIT. It simply doesn't apply to a plain L1 guest today, because such a guest isn't running SNP_INIT.

Thanks,
Ashish

> Jethro Beekman | CTO | Fortanix
> 
> On 2026-06-30 20:11, Ashish Kalra wrote:
>> From: Ashish Kalra <ashish.kalra@amd.com>
>>
>> While SNP is active, every memory write is checked against the RMP to
>> protect SEV-SNP guest memory.  A core performs these RMP checks only once
>> SNP has been initialized via SNP_INIT and the SNP-enable bit in SYSCFG is
>> set on that core; the firmware requires the SNP-enable bit to be set on
>> every present CPU before SNP initialization.  A core that is not
>> SNP-enabled and not SNP-initialized performs no RMP checks at all, so
>> there is no valid configuration with SNP active and any CPU exempt from
>> RMP checks.
>>
>> The firmware determines which CPUs are present from the processor and the
>> BIOS/UEFI configuration (e.g. SMT disabled in the BIOS) and enumerates
>> them at SNP init; it is not aware of the OS bringing CPUs online or
>> offline afterwards.  SNP_INIT fails unless SnpEn is set on all CPUs, so a
>> CPU that is offline at SNP init does not have SnpEn set, SNP_INIT fails,
>> and there can be no SNP guest memory.  OS CPU hotplug can thus diverge
>> from the firmware's expectations and break SNP.
>>
>> Tie CPU hotplug to the SNP-enable bit: disable it in snp_prepare() before
>> SNP is enabled, and re-enable it in snp_shutdown() once the firmware has
>> disabled SNP.  If snp_prepare() fails before enabling SNP it re-enables
>> hotplug itself; once SNP is enabled hotplug stays disabled, including
>> across a failed SNP_INIT and across the legacy SNP_SHUTDOWN_EX path, both
>> of which leave SNP enabled.  A kexec target that boots with SNP already
>> enabled disables hotplug once in snp_rmptable_init(), since snp_prepare()
>> bails when SNP is already enabled.
>>
>> This also keeps the CPU set stable for the asynchronous RMPOPT scan added
>> later in this series, and ensures cpus_read_lock() in the scan is
>> uncontended.
>>
>> Suggested-by: Thomas Lendacky <thomas.lendacky@amd.com>
>> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
>> ---
>>  arch/x86/virt/svm/sev.c | 31 +++++++++++++++++++++++++++++++
>>  1 file changed, 31 insertions(+)
>>
>> diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
>> index dab6e1c290bc..04a58ac4339c 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();
>> +
>>  	cpus_read_lock();
>>  
>>  	if (!cpumask_equal(cpu_online_mask, cpu_present_mask)) {
>> @@ -560,6 +569,10 @@ int snp_prepare(void)
>>  unlock:
>>  	cpus_read_unlock();
>>  
>> +	/* Re-enable CPU hotplug; SnpEn was never set. */
>> +	if (ret)
>> +		cpu_hotplug_enable();
>> +
>>  	return ret;
>>  }
>>  EXPORT_SYMBOL_FOR_MODULES(snp_prepare, "ccp");
>> @@ -587,6 +600,13 @@ void snp_shutdown(void)
>>  
>>  	rmpopt_cleanup();
>>  
>> +	/*
>> +	 * Re-enable CPU hotplug now that the firmware has disabled SNP; CPU
>> +	 * hotplug is not re-enabled for a legacy SNP shutdown.  After
>> +	 * rmpopt_cleanup() so RMPOPT_BASE is cleared with hotplug still disabled.
>> +	 */
>> +	cpu_hotplug_enable();
>> +
>>  	clear_rmp();
>>  	on_each_cpu(mfd_reconfigure, NULL, 1);
>>  }
>> @@ -645,6 +665,8 @@ EXPORT_SYMBOL_FOR_MODULES(snp_setup_rmpopt, "ccp");
>>   */
>>  int __init snp_rmptable_init(void)
>>  {
>> +	u64 val;
>> +
>>  	if (WARN_ON_ONCE(!cc_platform_has(CC_ATTR_HOST_SEV_SNP)))
>>  		return -ENOSYS;
>>  
>> @@ -654,6 +676,15 @@ int __init snp_rmptable_init(void)
>>  	if (!setup_rmptable())
>>  		return -ENOSYS;
>>  
>> +	/*
>> +	 * On a kexec boot SNP may already be enabled (legacy firmware leaves
>> +	 * SnpEn set across shutdown), in which case snp_prepare() bails without
>> +	 * disabling CPU hotplug, so disable it here.
>> +	 */
>> +	rdmsrq(MSR_AMD64_SYSCFG, val);
>> +	if (val & MSR_AMD64_SYSCFG_SNP_EN)
>> +		cpu_hotplug_disable();
>> +
>>  	/*
>>  	 * Setting crash_kexec_post_notifiers to 'true' to ensure that SNP panic
>>  	 * notifier is invoked to do SNP IOMMU shutdown before kdump.
>