Copy the initial CPUID from plane 0.  To avoid mismatches, block
KVM_SET_CPUID{,2} after KVM_CREATE_VCPU_PLANE similar to how it is
blocked after KVM_RUN; this is handled by a tiny bit of architecture
independent code.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 Documentation/virt/kvm/api.rst |  4 +++-
 arch/x86/kvm/cpuid.c           | 19 ++++++++++++++++++-
 arch/x86/kvm/cpuid.h           |  1 +
 arch/x86/kvm/x86.c             |  7 ++++++-
 include/linux/kvm_host.h       |  1 +
 virt/kvm/kvm_main.c            |  1 +
 6 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 16d836b954dc..3739d16b7164 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -736,7 +736,9 @@ Caveat emptor:
     configuration (if there is) is not corrupted. Userspace can get a copy
     of the resulting CPUID configuration through KVM_GET_CPUID2 in case.
   - Using KVM_SET_CPUID{,2} after KVM_RUN, i.e. changing the guest vCPU model
-    after running the guest, may cause guest instability.
+    after running the guest, is forbidden; so is using the ioctls after
+    KVM_CREATE_VCPU_PLANE, because all planes must have the same CPU
+    capabilities.
   - Using heterogeneous CPUID configurations, modulo APIC IDs, topology, etc...
     may cause guest instability.
 
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 142decb3a736..44e6d4989bdd 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -545,7 +545,7 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
 	 * KVM_SET_CPUID{,2} again. To support this legacy behavior, check
 	 * whether the supplied CPUID data is equal to what's already set.
 	 */
-	if (kvm_vcpu_has_run(vcpu)) {
+	if (kvm_vcpu_has_run(vcpu) || vcpu->has_planes) {
 		r = kvm_cpuid_check_equal(vcpu, e2, nent);
 		if (r)
 			goto err;
@@ -567,6 +567,23 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
 	return r;
 }
 
+int kvm_dup_cpuid(struct kvm_vcpu *vcpu, struct kvm_vcpu *source)
+{
+	if (WARN_ON_ONCE(vcpu->arch.cpuid_entries || vcpu->arch.cpuid_nent))
+		return -EEXIST;
+
+	vcpu->arch.cpuid_entries = kmemdup(source->arch.cpuid_entries,
+		     source->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2),
+		     GFP_KERNEL_ACCOUNT);
+	if (!vcpu->arch.cpuid_entries)
+		return -ENOMEM;
+
+	memcpy(vcpu->arch.cpu_caps, source->arch.cpu_caps, sizeof(source->arch.cpu_caps));
+	vcpu->arch.cpuid_nent = source->arch.cpuid_nent;
+
+	return 0;
+}
+
 /* when an old userspace process fills a new kernel module */
 int kvm_vcpu_ioctl_set_cpuid(struct kvm_vcpu *vcpu,
 			     struct kvm_cpuid *cpuid,
diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index 05cc1245f570..a5983c635a70 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -42,6 +42,7 @@ static inline struct kvm_cpuid_entry2 *kvm_find_cpuid_entry(struct kvm_vcpu *vcp
 int kvm_dev_ioctl_get_cpuid(struct kvm_cpuid2 *cpuid,
 			    struct kvm_cpuid_entry2 __user *entries,
 			    unsigned int type);
+int kvm_dup_cpuid(struct kvm_vcpu *vcpu, struct kvm_vcpu *source);
 int kvm_post_set_cpuid(struct kvm_vcpu *vcpu);
 int kvm_vcpu_ioctl_set_cpuid(struct kvm_vcpu *vcpu,
 			     struct kvm_cpuid *cpuid,
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d2b43d9b6543..be4d7b97367b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -12412,6 +12412,11 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu, struct kvm_plane *plane)
 	if (plane->plane) {
 		page = NULL;
 		vcpu->arch.pio_data = vcpu->plane0->arch.pio_data;
+		r = kvm_dup_cpuid(vcpu, vcpu->plane0);
+		if (r < 0)
+			goto fail_free_lapic;
+
+		r = -ENOMEM;
 	} else {
 		page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
 		if (!page)
@@ -12459,7 +12464,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu, struct kvm_plane *plane)
 
 	kvm_xen_init_vcpu(vcpu);
 	vcpu_load(vcpu);
-	kvm_vcpu_after_set_cpuid(vcpu);
+	WARN_ON_ONCE(kvm_post_set_cpuid(vcpu));
 	kvm_set_tsc_khz(vcpu, vcpu->kvm->arch.default_tsc_khz);
 	kvm_vcpu_reset(vcpu, false);
 	kvm_init_mmu(vcpu);
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 5cade1c04646..0b764951f461 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -344,6 +344,7 @@ struct kvm_vcpu {
 	struct mutex mutex;
 
 	/* Only valid on plane 0 */
+	bool has_planes;
 	bool wants_to_run;
 
 	/* Shared for all planes */
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index db38894f6fa3..3a04fdf0865d 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4182,6 +4182,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm_plane *plane, struct kvm_vcpu *pl
 	if (plane->plane) {
 		page = NULL;
 		vcpu->run = plane0_vcpu->run;
+		plane0_vcpu->has_planes = true;
 	} else {
 		WARN_ON(plane0_vcpu != NULL);
 		plane0_vcpu = vcpu;
-- 
2.49.0