[PATCH v2 11/49] KVM: x86: Disallow KVM_CAP_X86_DISABLE_EXITS after vCPU creation

[Sean Christopherson]

Reject KVM_CAP_X86_DISABLE_EXITS if vCPUs have been created, as disabling
PAUSE/MWAIT/HLT exits after vCPUs have been created is broken and useless,
e.g. except for PAUSE on SVM, the relevant intercepts aren't updated after
vCPU creation.  vCPUs may also end up with an inconsistent configuration
if exits are disabled between creation of multiple vCPUs.

Cc: Hou Wenlong <houwenlong.hwl@xxxxxxxxxxxx>
Link: https://lore.kernel.org/all/9227068821b275ac547eb2ede09ec65d2281fe07.1680179693.git.houwenlong.hwl@xxxxxxxxxxxx
Link: https://lore.kernel.org/all/20230121020738.2973-2-kechenl@xxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 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 6ab8b5b7c64e..884846282d06 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7645,6 +7645,7 @@ 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 vCPUs have already been created
 
 Valid bits in args[0] are::
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index bb34891d2f0a..4cb0c150a2f8 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6568,6 +6568,10 @@ 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_PAUSE)
 			kvm->arch.pause_in_guest = true;
 
@@ -6589,6 +6593,8 @@ 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];
-- 
2.45.0.215.g3402c0e53f-goog