[PATCH v9 8/9] KVM: x86: Allow userspace set maximum VCPU id for VM

[Zeng Guang <guang.zeng@intel.com>]

Introduce new max_vcpu_ids in KVM for x86 architecture. Userspace
can assign maximum possible vcpu id for current VM session using
KVM_CAP_MAX_VCPU_ID of KVM_ENABLE_CAP ioctl().

This is done for x86 only because the sole use case is to guide
memory allocation for PID-pointer table, a structure needed to
enable VMX IPI.

By default, max_vcpu_ids set as KVM_MAX_VCPU_IDS.

Suggested-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Zeng Guang <guang.zeng@intel.com>
---
 Documentation/virt/kvm/api.rst  | 18 ++++++++++++++++++
 arch/x86/include/asm/kvm_host.h |  6 ++++++
 arch/x86/kvm/x86.c              | 25 ++++++++++++++++++++++++-
 3 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index d13fa6600467..0c6ad2d8bea0 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7136,6 +7136,24 @@ The valid bits in cap.args[0] are:
                                     IA32_MISC_ENABLE[bit 18] is cleared.
 =================================== ============================================
 
+7.32 KVM_CAP_MAX_VCPU_ID
+------------------------
+
+:Architectures: x86
+:Target: VM
+:Parameters: args[0] - maximum APIC ID value set for current VM
+:Returns: 0 on success, -EINVAL if args[0] is beyond KVM_MAX_VCPU_IDS
+          supported in KVM or if it has been settled.
+
+Userspace is able to calculate the limit to APIC ID values from designated CPU
+topology. This capability allows userspace to specify maximum possible APIC ID
+assigned for current VM session prior to the creation of vCPUs. By design, it
+can set only once and doesn't accept change any more. KVM will manage memory
+allocation of VM-scope structures which depends on the value of APIC ID.
+
+Calling KVM_CHECK_EXTENSION for this capability returns the value of maximum APIC
+ID that KVM supports at runtime. It sets as KVM_MAX_VCPU_IDS by default.
+
 8. Other capabilities.
 ======================
 
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index d23e80a56eb8..cdd14033988d 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1238,6 +1238,12 @@ struct kvm_arch {
 	hpa_t	hv_root_tdp;
 	spinlock_t hv_root_tdp_lock;
 #endif
+	/*
+	 * VM-scope maximum vCPU ID. Used to determine the size of structures
+	 * that increase along with the maximum vCPU ID, in which case, using
+	 * the global KVM_MAX_VCPU_IDS may lead to significant memory waste.
+	 */
+	u32 max_vcpu_ids;
 };
 
 struct kvm_vm_stat {
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 277a0da8c290..744e88a71b63 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4320,7 +4320,10 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		r = KVM_MAX_VCPUS;
 		break;
 	case KVM_CAP_MAX_VCPU_ID:
-		r = KVM_MAX_VCPU_IDS;
+		if (!kvm->arch.max_vcpu_ids)
+			r = KVM_MAX_VCPU_IDS;
+		else
+			r = kvm->arch.max_vcpu_ids;
 		break;
 	case KVM_CAP_PV_MMU:	/* obsolete */
 		r = 0;
@@ -6064,6 +6067,20 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		}
 		mutex_unlock(&kvm->lock);
 		break;
+	case KVM_CAP_MAX_VCPU_ID:
+		r = -EINVAL;
+		if (cap->args[0] > KVM_MAX_VCPU_IDS)
+			break;
+
+		mutex_lock(&kvm->lock);
+		if (kvm->arch.max_vcpu_ids == cap->args[0]) {
+			r = 0;
+		} else if (!kvm->arch.max_vcpu_ids) {
+			kvm->arch.max_vcpu_ids = cap->args[0];
+			r = 0;
+		}
+		mutex_unlock(&kvm->lock);
+		break;
 	default:
 		r = -EINVAL;
 		break;
@@ -11172,6 +11189,12 @@ int kvm_arch_vcpu_precreate(struct kvm *kvm, unsigned int id)
 		pr_warn_once("kvm: SMP vm created on host with unstable TSC; "
 			     "guest TSC will not be reliable\n");
 
+	if (!kvm->arch.max_vcpu_ids)
+		kvm->arch.max_vcpu_ids = KVM_MAX_VCPU_IDS;
+
+	if (id >= kvm->arch.max_vcpu_ids)
+		return -EINVAL;
+
 	return 0;
 }
 
-- 
2.27.0