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

[Kalra, Ashish]

Hi Prateek,

On 7/1/2026 11:39 AM, K Prateek Nayak wrote:
> Hello Jethro,
> 
> On 7/1/2026 3:10 PM, Jethro Beekman wrote:
>> I don't believe my concern has been addressed
>>
>> https://lore.kernel.org/lkml/0df3b665-3a9c-4c46-a7aa-14388e8e1577@xxxxxxxxxxxx/
> 
> Quoting your question:
> 
>> I think this is too broad. If I have a hypervisor that supports SNP
>> virtualization, a (non-confidential) L1 guest running Linux should
>> still support CPU hotplug while also running confidential L2 guests.
> 
> Ashish, Tom, correct me if I'm wrong, but I don't think KVM exposes SNP
> support to L1, at least as per
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kvm/cpuid.c?h=v7.2-rc1#n1221
> and only SNP initialization disables hotplug - not the other variants.
> 
> L1, running a confidential guest (SEV/SEV-ES) should still be able to
> support hotplug since it doesn't go through SNP init. Only the base
> hypervisor can setup the RMP tables and go through snp_prepare().
> 
> Also bsp_determine_snp() should clear CC_ATTR_HOST_SEV_SNP if it
> detects X86_FEATURE_HYPERVISOR so I don't see how this can be a
> problem for hotplug in L1.
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/cpu/amd.c?h=v7.2-rc1#n368
> 

bsp_determine_snp() only sets CC_ATTR_HOST_SEV_SNP when X86_FEATURE_HYPERVISOR is clear:

  if (!cpu_has(c, X86_FEATURE_HYPERVISOR) &&
      (ZEN3 || ZEN4 || RMPREAD) && snp_probe_rmptable_info())
          cc_platform_set(CC_ATTR_HOST_SEV_SNP);
  else {
          setup_clear_cpu_cap(X86_FEATURE_SEV_SNP);
          cc_platform_clear(CC_ATTR_HOST_SEV_SNP);  
  }

So Linux running as an L1 guest (HYPERVISOR set) never has CC_ATTR_HOST_SEV_SNP.

And both hotplug-disable sites sit behind that flag:
  - snp_prepare() is only called from __sev_snp_init_locked(), which returns -ENODEV early if !cc_platform_has(CC_ATTR_HOST_SEV_SNP).
  - snp_rmptable_init() bails (WARN_ON_ONCE(!cc_platform_has(CC_ATTR_HOST_SEV_SNP))) before its kexec one-shot disable.

So an L1 guest can't reach the disable at all; only the bare-metal host that programs the RMP does.

An L1 running SEV/SEV-ES guests never goes through SNP host init, so it's hotplug is unaffected and KVM doesn't expose SNP to L1.

So there's no impact on L1 hotplug currently.

Thanks,
Ashish