On Tue, Apr 16, 2024, Ravi Bangoria wrote: > Currently, LBR Virtualization is dynamically enabled and disabled for > a vcpu by intercepting writes to MSR_IA32_DEBUGCTLMSR. This helps by > avoiding unnecessary save/restore of LBR MSRs when nobody is using it > in the guest. However, SEV-ES guest mandates LBR Virtualization to be > _always_ ON[1] and thus this dynamic toggling doesn't work for SEV-ES > guest, in fact it results into fatal error: > > SEV-ES guest on Zen3, kvm-amd.ko loaded with lbrv=1 > > [guest ~]# wrmsr 0x1d9 0x4 > KVM: entry failed, hardware error 0xffffffff > EAX=00000004 EBX=00000000 ECX=000001d9 EDX=00000000 > ... > > Fix this by never intercepting MSR_IA32_DEBUGCTLMSR for SEV-ES guests. Uh, what? I mean, sure, it works, maybe, I dunno. But there's a _massive_ disconnect between the first paragraph and this statement. Oh, good gravy, it "works" because SEV already forces LBR virtualization. svm->vmcb->control.virt_ext |= LBR_CTL_ENABLE_MASK; (a) the changelog needs to call that out. (b) KVM needs to disallow SEV-ES if LBR virtualization is disabled by the admin, i.e. if lbrv=false. Alternatively, I would be a-ok simply deleting lbrv, e.g. to avoid yet more printks about why SEV-ES couldn't be enabled. Hmm, I'd probably be more than ok. Because AMD (thankfully, blessedly) uses CPUID bits for SVM features, the admin can disable LBRV via clear_cpuid (or whatever it's called now). And there are hardly any checks on the feature, so it's not like having a boolean saves anything. AMD is clearly committed to making sure LBRV works, so the odds of KVM really getting much value out of a module param is low. And then when you delete lbrv, please add a WARN_ON_ONCE() sanity check in sev_hardware_setup() (if SEV-ES is supported), because like the DECODEASSISTS and FLUSHBYASID requirements, it's not super obvious that LBRV is a hard requirement for SEV-ES (that's an understatment; I'm curious how some decided that LBR virtualization is where the line go drawn for "yeah, _this_ is mandatory").
