linux-kernel - [RFC PATCH v2 1/3] KVM: x86: only allow exits disable before vCPUs created

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Date:   Tue, 21 Dec 2021 01:04:47 -0800
From:   Kechen Lu <kechenl@...dia.com>
To:     <kvm@...r.kernel.org>, <pbonzini@...hat.com>, <seanjc@...gle.com>
CC:     <wanpengli@...cent.com>, <vkuznets@...hat.com>, <mst@...hat.com>,
        <somduttar@...dia.com>, <kechenl@...dia.com>,
        <linux-kernel@...r.kernel.org>
Subject: [RFC PATCH v2 1/3] KVM: x86: only allow exits disable before vCPUs created

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.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Kechen Lu <kechenl@...dia.com>
---
 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 aeeb071c7688..d1c50b95bbc1 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6581,6 +6581,7 @@ 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 0cf1082455df..37529c0c279d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5764,6 +5764,10 @@ 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;
@@ -5774,6 +5778,8 @@ 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];
-- 
2.30.2