KVM: SVM: Fix dead KVM_BUG() code in LBR MSR virtualization

Refactor KVM's handling of LBR MSRs on SVM to avoid a second layer of
case statements, and thus eliminate a dead KVM_BUG() call, which (a) will
never be hit in the current code base and (b) if a future commit breaks
things, will never fire as KVM passes "false" instead "true" or '1' for
the KVM_BUG() condition.

Reported-by: Michal Luczaj <mhal@rbox.co>
Cc: Yuan Yao <yuan.yao@intel.com>
Link: https://lore.kernel.org/r/20230607203519.1570167-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 1bc0936..6239162 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -980,43 +980,22 @@ static void svm_disable_lbrv(struct kvm_vcpu *vcpu)
                svm_copy_lbrs(svm->vmcb01.ptr, svm->vmcb);
 }
 
-static int svm_get_lbr_msr(struct vcpu_svm *svm, u32 index)
+static struct vmcb *svm_get_lbr_vmcb(struct vcpu_svm *svm)
 {
        /*
-        * If the LBR virtualization is disabled, the LBR msrs are always
-        * kept in the vmcb01 to avoid copying them on nested guest entries.
-        *
-        * If nested, and the LBR virtualization is enabled/disabled, the msrs
-        * are moved between the vmcb01 and vmcb02 as needed.
+        * If LBR virtualization is disabled, the LBR MSRs are always kept in
+        * vmcb01.  If LBR virtualization is enabled and L1 is running VMs of
+        * its own, the MSRs are moved between vmcb01 and vmcb02 as needed.
         */
-       struct vmcb *vmcb =
-               (svm->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK) ?
-                       svm->vmcb : svm->vmcb01.ptr;
-
-       switch (index) {
-       case MSR_IA32_DEBUGCTLMSR:
-               return vmcb->save.dbgctl;
-       case MSR_IA32_LASTBRANCHFROMIP:
-               return vmcb->save.br_from;
-       case MSR_IA32_LASTBRANCHTOIP:
-               return vmcb->save.br_to;
-       case MSR_IA32_LASTINTFROMIP:
-               return vmcb->save.last_excp_from;
-       case MSR_IA32_LASTINTTOIP:
-               return vmcb->save.last_excp_to;
-       default:
-               KVM_BUG(false, svm->vcpu.kvm,
-                       "%s: Unknown MSR 0x%x", __func__, index);
-               return 0;
-       }
+       return svm->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK ? svm->vmcb :
+                                                                  svm->vmcb01.ptr;
 }
 
 void svm_update_lbrv(struct kvm_vcpu *vcpu)
 {
        struct vcpu_svm *svm = to_svm(vcpu);
 
-       bool enable_lbrv = svm_get_lbr_msr(svm, MSR_IA32_DEBUGCTLMSR) &
-                                          DEBUGCTLMSR_LBR;
+       bool enable_lbrv = svm_get_lbr_vmcb(svm)->save.dbgctl & DEBUGCTLMSR_LBR;
 
        bool current_enable_lbrv = !!(svm->vmcb->control.virt_ext &
                                      LBR_CTL_ENABLE_MASK);
@@ -2835,11 +2814,19 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
                msr_info->data = svm->tsc_aux;
                break;
        case MSR_IA32_DEBUGCTLMSR:
+               msr_info->data = svm_get_lbr_vmcb(svm)->save.dbgctl;
+               break;
        case MSR_IA32_LASTBRANCHFROMIP:
+               msr_info->data = svm_get_lbr_vmcb(svm)->save.br_from;
+               break;
        case MSR_IA32_LASTBRANCHTOIP:
+               msr_info->data = svm_get_lbr_vmcb(svm)->save.br_to;
+               break;
        case MSR_IA32_LASTINTFROMIP:
+               msr_info->data = svm_get_lbr_vmcb(svm)->save.last_excp_from;
+               break;
        case MSR_IA32_LASTINTTOIP:
-               msr_info->data = svm_get_lbr_msr(svm, msr_info->index);
+               msr_info->data = svm_get_lbr_vmcb(svm)->save.last_excp_to;
                break;
        case MSR_VM_HSAVE_PA:
                msr_info->data = svm->nested.hsave_msr;