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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