KVM: x86: Allow userspace to set maximum VCPU id for VM

mainline inclusion
from mainline-v6.0-rc1
commit 35875316
category: feature
feature: IPI Virtualization
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I5ODSC
CVE: N/A
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=35875316384b71d23dc2a45a969732fc8cab16af

Intel-SIG: commit 35875316 ("KVM: x86: Allow userspace to set maximum VCPU id for VM")

-------------------------------------

KVM: x86: Allow userspace to set maximum VCPU id for VM

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_ID.
Suggested-by: NSean Christopherson <seanjc@google.com>
Reviewed-by: NMaxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: NZeng Guang <guang.zeng@intel.com>
Message-Id: <20220419154444.11888-1-guang.zeng@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NJason Zeng <jason.zeng@intel.com>
Signed-off-by: NJason Zeng <jason.zeng@intel.com>

Documentation/virt/kvm/api.rst

@@ -6220,6 +6220,27 @@ default.
 
 See Documentation/x86/sgx.rst for more details.
 
+7.23 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_ID
+          supported in KVM or if it has been set.
+
+This capability allows userspace to specify maximum possible APIC ID
+assigned for current VM session prior to the creation of vCPUs, saving
+memory for data structures indexed by the APIC ID.  Userspace is able
+to calculate the limit to APIC ID values from designated
+CPU topology.
+
+The value can be changed only until KVM_ENABLE_CAP is set to a nonzero
+value or until a vCPU is created.  Upon creation of the first vCPU,
+if the value was set to zero or KVM_ENABLE_CAP was not invoked, KVM
+uses the return value of KVM_CHECK_EXTENSION(KVM_CAP_MAX_VCPU_ID) as
+the maximum APIC ID.
+
 8. Other capabilities.
 ======================
 

arch/x86/include/asm/kvm_host.h

@@ -1020,6 +1020,12 @@ struct kvm_arch {
 	struct list_head tdp_mmu_roots;
 	/* List of struct tdp_mmu_pages not being used as roots */
 	struct list_head tdp_mmu_pages;
+	/*
+	 * 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_ID may lead to significant memory waste.
+	 */
+	u32 max_vcpu_ids;
 };
 
 struct kvm_vm_stat {

arch/x86/kvm/x86.c

@@ -5432,6 +5432,20 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		break;
 	}
 #endif
+	case KVM_CAP_MAX_VCPU_ID:
+		r = -EINVAL;
+		if (cap->args[0] > KVM_MAX_VCPU_ID)
+			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;
@@ -10106,6 +10120,12 @@ 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_ID;
+
+	if (id >= kvm->arch.max_vcpu_ids)
+		return -EINVAL;
+
 	return 0;
 }