KVM: x86: Prevent excluding the BSP on setting max_vcpu_ids

If the BSP vCPU ID was already set, ensure it doesn't get excluded when
limiting vCPU IDs via KVM_CAP_MAX_VCPU_ID.

[mks: provide commit message, code by Sean]

Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Link: https://lore.kernel.org/r/20240614202859.3597745-4-minipli@grsecurity.net
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/x86.c

@@ -6707,7 +6707,9 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 			break;
 
 		mutex_lock(&kvm->lock);
-		if (kvm->arch.max_vcpu_ids == cap->args[0]) {
+		if (kvm->arch.bsp_vcpu_id > cap->args[0]) {
+			;
+		} else if (kvm->arch.max_vcpu_ids == cap->args[0]) {
 			r = 0;
 		} else if (!kvm->arch.max_vcpu_ids) {
 			kvm->arch.max_vcpu_ids = cap->args[0];