KVM: VMX: Compartmentalize adding MSRs to host vs. guest auto-load list

Undo the bundling of the "host" and "guest" MSR auto-load list logic so
that the code can be deduplicated by factoring out the logic to a separate
helper.  Now that "list full" situations are treated as fatal to the VM,
there is no need to pre-check both lists.

For all intents and purposes, this reverts the add_atomic_switch_msr()
changes made by commit 3190709 ("x86/KVM/VMX: Separate the VMX
AUTOLOAD guest/host number accounting").

Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Tested-by: Manali Shukla <manali.shukla@amd.com>
Link: https://patch.msgid.link/20251206001720.468579-42-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/vmx/vmx.c

@@ -1096,9 +1096,9 @@ static __always_inline void add_atomic_switch_msr_special(struct vcpu_vmx *vmx,
 static void add_atomic_switch_msr(struct vcpu_vmx *vmx, unsigned msr,
 				  u64 guest_val, u64 host_val)
 {
-	int i, j = 0;
 	struct msr_autoload *m = &vmx->msr_autoload;
 	struct kvm *kvm = vmx->vcpu.kvm;
+	int i;
 
 	switch (msr) {
 	case MSR_EFER:
@@ -1133,25 +1133,26 @@ static void add_atomic_switch_msr(struct vcpu_vmx *vmx, unsigned msr,
 	}
 
 	i = vmx_find_loadstore_msr_slot(&m->guest, msr);
-	j = vmx_find_loadstore_msr_slot(&m->host, msr);
-
-	if (KVM_BUG_ON(i < 0 && m->guest.nr == MAX_NR_LOADSTORE_MSRS, kvm) ||
-	    KVM_BUG_ON(j < 0 &&  m->host.nr == MAX_NR_LOADSTORE_MSRS, kvm))
-		return;
-
 	if (i < 0) {
+		if (KVM_BUG_ON(m->guest.nr == MAX_NR_LOADSTORE_MSRS, kvm))
+			return;
+
 		i = m->guest.nr++;
 		m->guest.val[i].index = msr;
 		vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT, m->guest.nr);
 	}
 	m->guest.val[i].value = guest_val;
 
-	if (j < 0) {
-		j = m->host.nr++;
-		m->host.val[j].index = msr;
+	i = vmx_find_loadstore_msr_slot(&m->host, msr);
+	if (i < 0) {
+		if (KVM_BUG_ON(m->host.nr == MAX_NR_LOADSTORE_MSRS, kvm))
+			return;
+
+		i = m->host.nr++;
+		m->host.val[i].index = msr;
 		vmcs_write32(VM_EXIT_MSR_LOAD_COUNT, m->host.nr);
 	}
-	m->host.val[j].value = host_val;
+	m->host.val[i].value = host_val;
 }
 
 static bool update_transition_efer(struct vcpu_vmx *vmx)