[PATCH 1/3] KVM: x86: Disallow enable KVM_CAP_X86_DISABLE_EXITS capability after vCPUs have been created

[Hou Wenlong]

Disable PAUSE/MWAIT/HLT exits after vCPUs have been created is useless,
because PAUSE/MWAIT/HLT intercepts configuration is not changed after
vCPU created.  And two vCPUs may have inconsistent configuration if
disable PAUSE/MWAIT/HLT exits between those two vCPUs creation. Since
it's a per-VM capability, all vCPUs should keep same configuration, so
disallow enable KVM_CAP_X86_DISABLE_EXITS capability after vCPUs have
been created.

Signed-off-by: Hou Wenlong <houwenlong.hwl@xxxxxxxxxxxx>
---
 Documentation/virt/kvm/api.rst | 3 ++-
 arch/x86/kvm/x86.c             | 5 +++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index a69e91088d76..95a683a27cf2 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7179,7 +7179,8 @@ 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
+:Returns: 0 on success, -EINVAL when args[0] contains invalid exits or
+	  any vCPUs have been created.
 
 Valid bits in args[0] are::
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2c0ff40e5345..7e97595465fc 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6275,6 +6275,9 @@ 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_PAUSE)
 			kvm->arch.pause_in_guest = true;
 
@@ -6296,6 +6299,8 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		}
 
 		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];

base-commit: 27d6845d258b67f4eb3debe062b7dacc67e0c393
-- 
2.31.1