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KVM: x86: Don't snapshot pending INIT/SIPI prior to checking nested events
Don't snapshot pending INIT/SIPI events prior to checking nested events, architecturally there's nothing wrong with KVM processing (dropping) a SIPI that is received immediately after synthesizing a VM-Exit. Taking and consuming the snapshot makes the flow way more subtle than it needs to be, e.g. nVMX consumes/clears events that trigger VM-Exit (INIT/SIPI), and so at first glance it appears that KVM is double-dipping on pending INITs and SIPIs. But that's not the case because INIT is blocked unconditionally in VMX root mode the CPU cannot be in wait-for_SIPI after VM-Exit, i.e. the paths that truly consume the snapshot are unreachable if apic->pending_events is modified by kvm_check_nested_events(). nSVM is a similar story as GIF is cleared by the CPU on VM-Exit; INIT is blocked regardless of whether or not it was pending prior to VM-Exit. Drop the snapshot logic so that a future fix doesn't create weirdness when kvm_vcpu_running()'s call to kvm_check_nested_events() is moved to vcpu_block(). In that case, kvm_check_nested_events() will be called immediately before kvm_apic_accept_events(), which raises the obvious question of why that change doesn't break the snapshot logic. Note, there is a subtle functional change. Previously, KVM would clear pending SIPIs if and only SIPI was pending prior to VM-Exit, whereas now KVM clears pending SIPI unconditionally if INIT+SIPI are blocked. The latter is architecturally allowed, as SIPI is ignored if the CPU is not in wait-for-SIPI mode (arguably, KVM should be even more aggressive in dropping SIPIs). It is software's responsibility to ensure the SIPI is delivered, i.e. software shouldn't be firing INIT-SIPI at a CPU until it knows with 100% certaining that the target CPU isn't in VMX root mode. Furthermore, the existing code is extra weird as SIPIs that arrive after VM-Exit _are_ dropped if there also happened to be a pending SIPI before VM-Exit. Signed-off-by:Sean Christopherson <seanjc@google.com> Message-Id: <20220921003201.1441511-10-seanjc@google.com> Signed-off-by: