KVM: x86: to track if L1 is running L2 VM

The new per-cpu stat 'nested_run' is introduced in order to track if L1 VM
is running or used to run L2 VM.

An example of the usage of 'nested_run' is to help the host administrator
to easily track if any L1 VM is used to run L2 VM. Suppose there is issue
that may happen with nested virtualization, the administrator will be able
to easily narrow down and confirm if the issue is due to nested
virtualization via 'nested_run'. For example, whether the fix like
commit 88dddc11 ("KVM: nVMX: do not use dangling shadow VMCS after
guest reset") is required.

Cc: Joe Jin <joe.jin@oracle.com>
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Message-Id: <20210305225747.7682-1-dongli.zhang@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -1125,6 +1125,7 @@ struct kvm_vcpu_stat {
 	u64 req_event;
 	u64 halt_poll_success_ns;
 	u64 halt_poll_fail_ns;
+	u64 nested_run;
 };
 
 struct x86_instruction_info;

arch/x86/kvm/svm/nested.c

@@ -494,6 +494,8 @@ int nested_svm_vmrun(struct vcpu_svm *svm)
 	struct kvm_host_map map;
 	u64 vmcb12_gpa;
 
+	++svm->vcpu.stat.nested_run;
+
 	if (is_smm(&svm->vcpu)) {
 		kvm_queue_exception(&svm->vcpu, UD_VECTOR);
 		return 1;

arch/x86/kvm/vmx/nested.c

@@ -3453,6 +3453,8 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
 	u32 interrupt_shadow = vmx_get_interrupt_shadow(vcpu);
 	enum nested_evmptrld_status evmptrld_status;
 
+	++vcpu->stat.nested_run;
+
 	if (!nested_vmx_check_permission(vcpu))
 		return 1;
 

arch/x86/kvm/x86.c

@@ -245,6 +245,7 @@ struct kvm_stats_debugfs_item debugfs_entries[] = {
 	VCPU_STAT("l1d_flush", l1d_flush),
 	VCPU_STAT("halt_poll_success_ns", halt_poll_success_ns),
 	VCPU_STAT("halt_poll_fail_ns", halt_poll_fail_ns),
+	VCPU_STAT("nested_run", nested_run),
 	VM_STAT("mmu_shadow_zapped", mmu_shadow_zapped),
 	VM_STAT("mmu_pte_write", mmu_pte_write),
 	VM_STAT("mmu_pde_zapped", mmu_pde_zapped),