[PATCH v3 1/8] KVM: X86: Reset vcpu->arch.cpuid_nent to 0 if SET_CPUID* fails

[Xiaoyao Li]

Current implementation keeps userspace input of CPUID configuration and
cpuid->nent even if kvm_update_cpuid() fails. Reset vcpu->arch.cpuid_nent
to 0 for the case of failure as a simple fix.

Besides, update the doc to explicitly state that if IOCTL SET_CPUID*
fail KVM gives no gurantee that previous valid CPUID configuration is
kept.

Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
---
 Documentation/virt/kvm/api.rst | 4 ++++
 arch/x86/kvm/cpuid.c           | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 1cfe79b932d6..3ca809a1a44f 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -669,6 +669,10 @@ MSRs that have been set successfully.
 Defines the vcpu responses to the cpuid instruction.  Applications
 should use the KVM_SET_CPUID2 ioctl if available.
 
+Note, when this IOCTL fails, KVM gives no guarantees that previous valid CPUID
+configuration (if there is) is not corrupted. Userspace can get a copy of valid
+CPUID configuration through KVM_GET_CPUID2 in case.
+
 ::
 
   struct kvm_cpuid_entry {
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 8a294f9747aa..1d13bad42bf9 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -207,6 +207,8 @@ int kvm_vcpu_ioctl_set_cpuid(struct kvm_vcpu *vcpu,
 	kvm_apic_set_version(vcpu);
 	kvm_x86_ops.cpuid_update(vcpu);
 	r = kvm_update_cpuid(vcpu);
+	if (r)
+		vcpu->arch.cpuid_nent = 0;
 
 	kvfree(cpuid_entries);
 out:
@@ -230,6 +232,8 @@ int kvm_vcpu_ioctl_set_cpuid2(struct kvm_vcpu *vcpu,
 	kvm_apic_set_version(vcpu);
 	kvm_x86_ops.cpuid_update(vcpu);
 	r = kvm_update_cpuid(vcpu);
+	if (r)
+		vcpu->arch.cpuid_nent = 0;
 out:
 	return r;
 }
-- 
2.18.4