[RFC PATCH v5 1/6] KVM: x86: only allow exits disable before vCPUs created

[Kechen Lu <kechenl@nvidia.com>]

From: Sean Christopherson <seanjc@google.com>

Since VMX and SVM both would never update the control bits if exits
are disable after vCPUs are created, only allow setting exits
disable flag before vCPU creation.

Fixes: 4d5422cea3b6 ("KVM: X86: Provide a capability to disable MWAIT
intercepts")

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Kechen Lu <kechenl@nvidia.com>
Cc: stable@vger.kernel.org
---
 Documentation/virt/kvm/api.rst | 1 +
 arch/x86/kvm/x86.c             | 6 ++++++
 2 files changed, 7 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 9807b05a1b57..fb0fcc566d5a 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7087,6 +7087,7 @@ branch to guests' 0x200 interrupt vector.
 :Architectures: x86
 :Parameters: args[0] defines which exits are disabled
 :Returns: 0 on success, -EINVAL when args[0] contains invalid exits
+          or if any vCPU has already been created
 
 Valid bits in args[0] are::
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index da4bbd043a7b..c8ae9c4f9f08 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6227,6 +6227,10 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		if (cap->args[0] & ~KVM_X86_DISABLE_VALID_EXITS)
 			break;
 
+		mutex_lock(&kvm->lock);
+		if (kvm->created_vcpus)
+			goto disable_exits_unlock;
+
 		if ((cap->args[0] & KVM_X86_DISABLE_EXITS_MWAIT) &&
 			kvm_can_mwait_in_guest())
 			kvm->arch.mwait_in_guest = true;
@@ -6237,6 +6241,8 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		if (cap->args[0] & KVM_X86_DISABLE_EXITS_CSTATE)
 			kvm->arch.cstate_in_guest = true;
 		r = 0;
+disable_exits_unlock:
+		mutex_unlock(&kvm->lock);
 		break;
 	case KVM_CAP_MSR_PLATFORM_INFO:
 		kvm->arch.guest_can_read_msr_platform_info = cap->args[0];
-- 
2.34.1