[RFC PATCH v6 4/6] KVM: x86: Let userspace re-enable previously disabled exits

[Kechen Lu <kechenl@nvidia.com>]

From: Sean Christopherson <seanjc@google.com>

Add an OVERRIDE flag to KVM_CAP_X86_DISABLE_EXITS allow userspace to
re-enable exits and/or override previous settings.  There's no real use
case for the per-VM ioctl, but a future per-vCPU variant wants to let
userspace toggle interception while the vCPU is running; add the OVERRIDE
functionality now to provide consistent between the per-VM and
per-vCPU variants.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 Documentation/virt/kvm/api.rst |  5 +++++
 arch/x86/kvm/x86.c             | 32 ++++++++++++++++++++++++--------
 include/uapi/linux/kvm.h       |  4 +++-
 3 files changed, 32 insertions(+), 9 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index fb0fcc566d5a..3850202942d0 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7095,6 +7095,7 @@ Valid bits in args[0] are::
   #define KVM_X86_DISABLE_EXITS_HLT              (1 << 1)
   #define KVM_X86_DISABLE_EXITS_PAUSE            (1 << 2)
   #define KVM_X86_DISABLE_EXITS_CSTATE           (1 << 3)
+  #define KVM_X86_DISABLE_EXITS_OVERRIDE         (1ull << 63)
 
 Enabling this capability on a VM provides userspace with a way to no
 longer intercept some instructions for improved latency in some
@@ -7103,6 +7104,10 @@ physical CPUs.  More bits can be added in the future; userspace can
 just pass the KVM_CHECK_EXTENSION result to KVM_ENABLE_CAP to disable
 all such vmexits.
 
+By default, this capability only disables exits.  To re-enable an exit, or to
+override previous settings, userspace can set KVM_X86_DISABLE_EXITS_OVERRIDE,
+in which case KVM will enable/disable according to the mask (a '1' == disable).
+
 Do not enable KVM_FEATURE_PV_UNHALT if you disable HLT exits.
 
 7.14 KVM_CAP_S390_HPAGE_1M
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 60caa3fd40e5..3ea5f12536a0 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5484,6 +5484,28 @@ static int kvm_vcpu_ioctl_device_attr(struct kvm_vcpu *vcpu,
 	return r;
 }
 
+
+#define kvm_ioctl_disable_exits(a, mask)				     \
+({									     \
+	if (!kvm_can_mwait_in_guest())                                       \
+		(mask) &= KVM_X86_DISABLE_EXITS_MWAIT;                       \
+	if ((mask) & KVM_X86_DISABLE_EXITS_OVERRIDE) {			     \
+		(a).mwait_in_guest = (mask) & KVM_X86_DISABLE_EXITS_MWAIT;   \
+		(a).hlt_in_guest = (mask) & KVM_X86_DISABLE_EXITS_HLT;	     \
+		(a).pause_in_guest = (mask) & KVM_X86_DISABLE_EXITS_PAUSE;   \
+		(a).cstate_in_guest = (mask) & KVM_X86_DISABLE_EXITS_CSTATE; \
+	} else {							     \
+		if ((mask) & KVM_X86_DISABLE_EXITS_MWAIT)		     \
+			(a).mwait_in_guest = true;			     \
+		if ((mask) & KVM_X86_DISABLE_EXITS_HLT)			     \
+			(a).hlt_in_guest = true;			     \
+		if ((mask) & KVM_X86_DISABLE_EXITS_PAUSE)		     \
+			(a).pause_in_guest = true;			     \
+		if ((mask) & KVM_X86_DISABLE_EXITS_CSTATE)		     \
+			(a).cstate_in_guest = true;			     \
+	}								     \
+})
+
 static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
 				     struct kvm_enable_cap *cap)
 {
@@ -6238,14 +6260,8 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		if (kvm->created_vcpus)
 			goto disable_exits_unlock;
 
-		if (cap->args[0] & KVM_X86_DISABLE_EXITS_MWAIT)
-			kvm->arch.mwait_in_guest = true;
-		if (cap->args[0] & KVM_X86_DISABLE_EXITS_HLT)
-			kvm->arch.hlt_in_guest = true;
-		if (cap->args[0] & KVM_X86_DISABLE_EXITS_PAUSE)
-			kvm->arch.pause_in_guest = true;
-		if (cap->args[0] & KVM_X86_DISABLE_EXITS_CSTATE)
-			kvm->arch.cstate_in_guest = true;
+		kvm_ioctl_disable_exits(kvm->arch, cap->args[0]);
+
 		r = 0;
 disable_exits_unlock:
 		mutex_unlock(&kvm->lock);
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 55155e262646..876dcccbfff2 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -823,10 +823,12 @@ struct kvm_ioeventfd {
 #define KVM_X86_DISABLE_EXITS_HLT            (1 << 1)
 #define KVM_X86_DISABLE_EXITS_PAUSE          (1 << 2)
 #define KVM_X86_DISABLE_EXITS_CSTATE         (1 << 3)
+#define KVM_X86_DISABLE_EXITS_OVERRIDE	     (1ull << 63)
 #define KVM_X86_DISABLE_VALID_EXITS          (KVM_X86_DISABLE_EXITS_MWAIT | \
                                               KVM_X86_DISABLE_EXITS_HLT | \
                                               KVM_X86_DISABLE_EXITS_PAUSE | \
-                                              KVM_X86_DISABLE_EXITS_CSTATE)
+					      KVM_X86_DISABLE_EXITS_CSTATE | \
+					      KVM_X86_DISABLE_EXITS_OVERRIDE)
 
 /* for KVM_ENABLE_CAP */
 struct kvm_enable_cap {
-- 
2.34.1