On 7/16/21 2:31 PM, Sean Christopherson wrote: > That's not what I was asking. My question is if KVM will break/fail if someone > runs a KVM build with SNP enabled halfway through the series. E.g. if I make a > KVM build at patch 22, "KVM: SVM: Add KVM_SNP_INIT command", what will happen if > I attempt to launch an SNP guest? Obviously it won't fully succeed, but will KVM > fail gracefully and do all the proper cleanup? Repeat the question for all patches > between this one and the final patch of the series. > > SNP simply not working is ok, but if KVM explodes or does weird things without > "full" SNP support, then at minimum the module param should be off by default > until it's safe to enable. E.g. for the TDP MMU, I believe the approach was to > put all the machinery in place but not actually let userspace flip on the module > param until the full implementation was ready. Bisecting and testing the > individual commits is a bit painful because it requires modifying KVM code, but > on the plus side unrelated bisects won't stumble into a half-baked state. There is one to two patches where I can think of that we may break the KVM if SNP guest is created before applying the full series. In one patch we add LAUNCH_UPDATE but reclaim is done in next patch. I like your idea to push the module init later in the series. > > Ya, got that, but again not what I was asking :-) Why use cpu_feature_enabled() > instead of boot_cpu_has()? As a random developer, I would fully expect that > boot_cpu_has(X86_FEATURE_SEV_SNP) is true iff SNP is fully enabled by the kernel. I have to check but I think boot_cpu_has(X64_FEATURE_SEV_SNP) will return true even when the CONFIG_MEM_ENCRYPT is disabled. > >> The approach here is similar to SEV/ES. IIRC, it was done mainly to >> avoid adding dead code when CONFIG_KVM_AMD_SEV is disabled. > But this is already in an #ifdef, checking sev_es_guest() is pointless. Ah Good point.
